Repository: yt-dlp
Issue: Add "postprocessing" section to readme
Labels: docs/meta/cleanup,PR-needed,core:post-processor

### Checklist

- [X] I'm reporting a bug unrelated to a specific site
- [X] I've verified that I'm running yt-dlp version **2022.02.04**. ([update instructions](https://github.com/yt-dlp/yt-dlp#update))
- [X] I've checked that all provided URLs are alive and playable in a browser
- [X] I've checked that all URLs and arguments with special characters are [properly quoted or escaped](https://github.com/ytdl-org/youtube-dl#video-url-contains-an-ampersand-and-im-getting-some-strange-output-1-2839-or-v-is-not-recognized-as-an-internal-or-external-command)
- [X] I've searched the [bugtracker](https://github.com/yt-dlp/yt-dlp/issues?q=) for similar issues including closed ones. DO NOT post duplicates
- [X] I've read the [guidelines for opening an issue](https://github.com/yt-dlp/yt-dlp/blob/master/CONTRIBUTING.md#opening-an-issue)

### Description

I am trying to download audio only and convert that audio to PCM.

For some reason, when a format argument (in my case, `-f s16le`) is specified in `--postprocessor-args`, m4a fixup (where it apparently adjusts the file to not be a `DASH` one or something like that) doesn't seem to actually happen, although it says it does and it doesn't give an error.

I've omitted the `--extract-audio --audio-format wav` stuff as that falls over at the end as a result, but I don't think it's connected to this as I can see the m4a file is clearly wrong after the first command has run.

Command 1 (with `-f s16le`)
```
yt-dlp -o "%(id)s.%(ext)s" -f bestaudio/best --postprocessor-args "ffmpeg:-ac 2 -f s16le -ar 48000" --compat-options youtube-dl https://www.youtube.com/watch?v=54rwawswQpM/
```
Command 2 (without `-f s16le`)
```
yt-dlp -o "%(id)s.%(ext)s" -f bestaudio/best --postprocessor-args "ffmpeg:-ac 2 -ar 48000" --compat-options youtube-dl https://www.youtube.com/watch?v=54rwawswQpM/
```

### Verbose log

```shell
Command 1 output:

[debug] Command-line config: ['-o', '%(id)s.%(ext)s', '-f', 'bestaudio/best', '--postprocessor-args', 'ffmpeg:-ac 2 -f s16le -ar 48000', '--compat-options', 'youtube-dl', 'https://www.youtube.com/watch?v=54rwawswQpM/', '-vU']
[debug] Encodings: locale cp1252, fs utf-8, out cp1252 (No ANSI), err cp1252 (No ANSI), pref cp1252
[debug] yt-dlp version 2022.02.04 [c1653e9] (win_exe)
[debug] Compatibility options: no-youtube-unavailable-videos, format-spec, seperate-video-versions, no-attach-info-json, playlist-index, no-youtube-channel-redirect, list-formats, embed-thumbnail-atomicparsley, no-clean-infojson, embed-metadata, no-direct-merge, *filename, no-playlist-metafiles, no-keep-subs, abort-on-error, format-sort, no-live-chat
[debug] Python version 3.8.10 (CPython 64bit) - Windows-8.1-6.3.9600-SP0
[debug] exe versions: ffmpeg 5.0-full_build-www.gyan.dev (setts), ffprobe 5.0-full_build-www.gyan.dev
[debug] Optional libraries: Cryptodome, mutagen, sqlite, websockets
[debug] Proxy map: {}
Latest version: 2022.02.04, Current version: 2022.02.04
yt-dlp is up to date (2022.02.04)
[debug] [youtube] Extracting URL: https://www.youtube.com/watch?v=54rwawswQpM/
[youtube] 54rwawswQpM: Downloading webpage
[youtube] 54rwawswQpM: Downloading android player API JSON
[debug] Sort order given by user: hasaud, lang, quality, tbr, filesize, vbr, height, width, proto, vext, abr, aext, fps, fs_approx, source, id
[debug] Sort order given by extractor: quality, res, fps, hdr:12, source, codec:vp9.2, lang, proto
[debug] Formats sorted by: hasvid, ie_pref, hasaud, lang, quality, tbr, filesize, vbr, height, width, proto, vext, abr, aext, fps, fs_approx, source, id, res, hdr:12(7), vcodec:vp9.2(10), acodec, asr
[info] 54rwawswQpM: Downloading 1 format(s): 140
[debug] Invoking downloader on "https://rr5---sn-aigl6nl7.googlevideo.com/videoplayback?<REDACTED>"
[download] Destination: 54rwawswQpM.m4a

[download]   0.0% of 5.29MiB at Unknown speed ETA Unknown
[download]   0.1% of 5.29MiB at Unknown speed ETA Unknown
[download]   0.1% of 5.29MiB at Unknown speed ETA Unknown
[download]   0.3% of 5.29MiB at  7.50MiB/s ETA 00:00     
[download]   0.6% of 5.29MiB at  3.87MiB/s ETA 00:01
[download]   1.2% of 5.29MiB at  4.85MiB/s ETA 00:01
[download]   2.3% of 5.29MiB at  6.05MiB/s ETA 00:00
[download]   4.7% of 5.29MiB at  7.97MiB/s ETA 00:00
[download]   9.4% of 5.29MiB at 11.61MiB/s ETA 00:00
[download]  18.9% of 5.29MiB at 15.98MiB/s ETA 00:00
[download]  37.8% of 5.29MiB at 18.95MiB/s ETA 00:00
[download]  75.6% of 5.29MiB at 18.36MiB/s ETA 00:00
[download] 100% of 5.29MiB at 18.43MiB/s ETA 00:00  
[download] 100% of 5.29MiB in 00:00               
[FixupM4a] Correcting container of "54rwawswQpM.m4a"
[debug] ffmpeg command line: ffmpeg -y -loglevel "repeat+info" -i "file:54rwawswQpM.m4a" -map 0 -dn -ignore_unknown -c copy -f mp4 -movflags "+faststart" -ac 2 -f s16le -ar 48000 "file:54rwawswQpM.temp.m4a"

Command 2 output:

[debug] Command-line config: ['-o', '%(id)s.%(ext)s', '-f', 'bestaudio/best', '--postprocessor-args', 'ffmpeg:-ac 2 -ar 48000', '--compat-options', 'youtube-dl', 'https://www.youtube.com/watch?v=54rwawswQpM/', '-vU']
[debug] Encodings: locale cp1252, fs utf-8, out cp1252 (No ANSI), err cp1252 (No ANSI), pref cp1252
[debug] yt-dlp version 2022.02.04 [c1653e9] (win_exe)
[debug] Compatibility options: no-attach-info-json, embed-metadata, embed-thumbnail-atomicparsley, no-youtube-unavailable-videos, no-clean-infojson, no-direct-merge, list-formats, no-youtube-channel-redirect, no-keep-subs, *filename, no-playlist-metafiles, seperate-video-versions, playlist-index, format-spec, no-live-chat, format-sort, abort-on-error
[debug] Python version 3.8.10 (CPython 64bit) - Windows-8.1-6.3.9600-SP0
[debug] exe versions: ffmpeg 5.0-full_build-www.gyan.dev (setts), ffprobe 5.0-full_build-www.gyan.dev
[debug] Optional libraries: Cryptodome, mutagen, sqlite, websockets
[debug] Proxy map: {}
Latest version: 2022.02.04, Current version: 2022.02.04
yt-dlp is up to date (2022.02.04)
[debug] [youtube] Extracting URL: https://www.youtube.com/watch?v=54rwawswQpM/
[youtube] 54rwawswQpM: Downloading webpage
[youtube] 54rwawswQpM: Downloading android player API JSON
[debug] Sort order given by user: hasaud, lang, quality, tbr, filesize, vbr, height, width, proto, vext, abr, aext, fps, fs_approx, source, id
[debug] Sort order given by extractor: quality, res, fps, hdr:12, source, codec:vp9.2, lang, proto
[debug] Formats sorted by: hasvid, ie_pref, hasaud, lang, quality, tbr, filesize, vbr, height, width, proto, vext, abr, aext, fps, fs_approx, source, id, res, hdr:12(7), vcodec:vp9.2(10), acodec, asr
[info] 54rwawswQpM: Downloading 1 format(s): 140
[debug] Invoking downloader on "https://rr5---sn-aigzrne7.googlevideo.com/videoplayback?<REDACTED>"
[download] Destination: 54rwawswQpM.m4a

[download]   0.0% of 5.29MiB at 341.28KiB/s ETA 00:15
[download]   0.1% of 5.29MiB at 1023.83KiB/s ETA 00:05
[download]   0.1% of 5.29MiB at  2.33MiB/s ETA 00:02  
[download]   0.3% of 5.29MiB at  5.00MiB/s ETA 00:01
[download]   0.6% of 5.29MiB at  3.87MiB/s ETA 00:01
[download]   1.2% of 5.29MiB at  4.85MiB/s ETA 00:01
[download]   2.3% of 5.29MiB at  6.05MiB/s ETA 00:00
[download]   4.7% of 5.29MiB at  7.73MiB/s ETA 00:00
[download]   9.4% of 5.29MiB at 11.61MiB/s ETA 00:00
[download]  18.9% of 5.29MiB at 15.50MiB/s ETA 00:00
[download]  37.8% of 5.29MiB at 17.20MiB/s ETA 00:00
[download]  75.6% of 5.29MiB at 17.80MiB/s ETA 00:00
[download] 100% of 5.29MiB at 17.94MiB/s ETA 00:00  
[download] 100% of 5.29MiB in 00:00               
[FixupM4a] Correcting container of "54rwawswQpM.m4a"
[debug] ffmpeg command line: ffmpeg -y -loglevel "repeat+info" -i "file:54rwawswQpM.m4a" -map 0 -dn -ignore_unknown -c copy -f mp4 -movflags "+faststart" -ac 2 -ar 48000 "file:54rwawswQpM.temp.m4a"
```