avformat/avformat.h: Correct some comments

avformat/avformat.h: Correct some comments

1. When set_parameters was removed from AVOutputFormat in 2fb75019, it
was forgotten to remove the comment pertaining to it. Said comment now
appeared to apply to interleave_packet(); it is of course nonsense and
has been replaced by an accurate description.
2. The description of av_write_uncoded_frame() suggested
av_interleaved_write_frame() as a replacement if the input is not
already correctly interleaved; it also referred to said function for
details. Given that said function can't write AVFrames and that the
specifics of writing uncoded frames are explained in the description
of av_interleaved_write_uncoded_frame(), both references have been fixed.
3. Removed an outdated comment about avformat_seek_file().

Reviewed-by: Marton Balint <cus@passwd.hu>
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>