[MPlayer-dev-eng] [DOCS] [3rd try] missing option descriptions

[Diego Biurrun]

Arpi writes:
 > > sub-demuxer
 > 
 > it's the same as audio-demuxer, but for subtitles.
 > since subtitles in g1 aren't real streams (yet), it has no much use.

Something like this:

  -sub-demuxer <number> (-subfile only)
    Force audio demuxer type for -subfile.  Give the demuxer ID as
    defined in  <subtitle_header_file>.h.

?

Diego