manpage: update --playlist comments

Using the --playlist option is no longer recommended.

A while ago, mpv rewrote all playlist parsers and added some minimal
security mechanisms (like not allowing local file access or unsafe
protocols in remote playlists). Further, mpv can load playlists by
passing them as normal file arguments, without the option.

Now, --playlist is needed only in these situations:
1) loading plaintext files
2) disabling additional security mechanisms
   (e.g. using a remote playlist to play local files)
player: add --chapters-file option

Note that you can't pass .cue or .edl files to it, at least not yet.

Requested in context of allowing to specify custom chapters. For that
to work well, we probably need to add some sort of chapter metadata
pseudo-demuxer.
player: always use demux_chapter

Instead of defining a separate data structure in the core.

For some odd reason, demux_chapter exported the chapter time in
nano-seconds. Change that to the usual timestamps (rename the field
to make any code relying on this to fail compilation), and also remove
the unused chapter end time.
player: print anamorphic size only if video is anamorphic

Has been annoying me since forever.
common: fix version variable declarations

This was stupid.
command: expose mpv version as property

A client API user has no other way to know the version.
client API: nothing uses the result parameter of mpv_command_node()

Add a comment about this to avoid confusing users of this function. The
parameter is essentially unused, but exists so that we don't need to
add extra APIs if the need for it arises.