doc/bistream_filters.texi: add documentation for the h264_mp4toannexb filter

doc/bitstream_filters.texi

@@ -23,6 +23,20 @@ Below is a description of the currently available bitstream filters.
 
 @section h264_mp4toannexb
 
+Convert an H.264 bitstream from length prefixed mode to start code
+prefixed mode (as defined in the Annex B of the ITU-T H.264
+specification).
+
+This is required by some streaming formats, typically the MPEG-2
+transport stream format ("mpegts").
+
+For example to remux an MP4 file containing an H.264 stream to mpegts
+format with @command{ffmpeg}, you can use the command:
+
+@example
+ffmpeg -i INPUT.mp4 -codec copy -bsf:v h264_mp4toannexb OUTPUT.ts
+@end example
+
 @section imx_dump_header
 
 @section mjpeg2jpeg