Fix documentation for "-debug" commandline argument

(cherry picked from commit 180e782942)

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

doc/ffmpeg.texi

@@ -712,8 +712,39 @@ ffmpeg -i in.ogg -map_metadata 0:0,s0 out.mp3
 Copy chapters from @var{infile} to @var{outfile}. If no chapter mapping is specified,
 then chapters are copied from the first input file with at least one chapter to all
 output files. Use a negative file index to disable any chapter copying.
-@item -debug
+@item -debug @var{category}
 Print specific debug info.
+@var{category} is a number or a string containing one of the following values:
+@table @samp
+@item bitstream
+@item buffers
+picture buffer allocations
+@item bugs
+@item dct_coeff
+@item er
+error recognition
+@item mb_type
+macroblock (MB) type
+@item mmco
+memory management control operations (H.264)
+@item mv
+motion vector
+@item pict
+picture info
+@item pts
+@item qp
+per-block quantization parameter (QP)
+@item rc
+rate control
+@item skip
+@item startcode
+@item thread_ops
+threading operations
+@item vis_mb_type
+visualize block types
+@item vis_qp
+visualize quantization parameter (QP), lower QP are tinted greener
+@end table
 @item -benchmark
 Show benchmarking information at the end of an encode.
 Shows CPU time used and maximum memory consumption.