doc/bitstream_filters: document dump_extra

doc/bitstream_filters.texi

@@ -32,6 +32,32 @@ Remove zero padding at the end of a packet.
 
 @section dump_extra
 
+Add extradata to the beginning of the filtered packets.
+
+The additional argument specifies which packets should be filtered.
+It accepts the values:
+@table @samp
+@item a
+add extradata to all key packets, but only if @var{local_header} is
+set in the @option{flags2} codec context field
+
+@item k
+add extradata to all key packets
+
+@item e
+add extradata to all packets
+@end table
+
+If not specified it is assumed @samp{k}.
+
+For example the following @command{ffmpeg} command forces a global
+header (thus disabling individual packet headers) in the H.264 packets
+generated by the @code{libx264} encoder, but corrects them by adding
+the header stored in extradata to the key packets:
+@example
+ffmpeg -i INPUT -map 0 -flags:v +global_header -c:v libx264 -bsf:v dump_extra out.ts
+@end example
+
 @section h264_mp4toannexb
 
 Convert an H.264 bitstream from length prefixed mode to start code