Also remove the (unused) AAC_AC3_PARSE_ERROR_CHANNEL_CFG while at it;
furthermore, fix the documentation of ff_ac3_parse_header()
and (ff|avpriv)_adts_header_parse().
Reviewed-by: Andrew Sayers <ffmpeg-devel@pileofstuff.org>
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@outlook.com>