Changed the mencoder_ass_defaultstyle setting to use_embedded_subtitles_style and added documentation

Now it applies to FFmpeg instead of just MEncoder, which is a better default because before this commit we were applying our own settings to ASS/SSA subtitles by default which is an advanced thing better left to user preference
Made FFmpeg use lower audio bitrates if there are only 1 or 2 audio channels
Use new settings
Saved bandwidth by preventing FFmpeg from upconverting 2-channel audio to 6-channel