doc/fftools-common-opts: document ffmpeg -h bsf=bitstream_filter_name

document ffmpeg -h bsf=bitstream_filter_name
Signed-off-by: Jun Zhao <barryjzhao@tencent.com>

doc/fftools-common-opts.texi

@@ -109,6 +109,10 @@ Print detailed information about the muxer named @var{muxer_name}. Use the
 @item filter=@var{filter_name}
 Print detailed information about the filter name @var{filter_name}. Use the
 @option{-filters} option to get a list of all filters.
+
+@item bsf=@var{bitstream_filter_name}
+Print detailed information about the bitstream filter name @var{bitstream_filter_name}.
+Use the @option{-bsfs} option to get a list of all bitstream filters.
 @end table
 
 @item -version