[MPlayer-dev-eng] [PATCH][DOCS] Update to slave mode documentation

[Diego Biurrun]

Mike Swieton writes:
 > This fixes a typo introduced in the previous patch I submitted (the
 > get_vo_fullscreen command) in the documentation, as well as adding
 > documentation to the existing get_percent_pos and get_time_length commands.

Thanks, applied.

Diego