doc/bitstream_filters: documents delete_filler option.

documents delete_filler option for bsf h264_metadata.
Signed-off-by: NJun Zhao <jun.zhao@intel.com>
Signed-off-by: NMichael Niedermayer <michael@niedermayer.cc>

doc/bitstream_filters.texi

@@ -153,6 +153,9 @@ possibly separated by hyphens, and the string can be anything.
 For example, @samp{086f3693-b7b3-4f2c-9653-21492feee5b8+hello} will
 insert the string ``hello'' associated with the given UUID.
 
+@item delete_filler
+Deletes both filler NAL units and filler SEI messages.
+
 @end table
 
 @section h264_mp4toannexb