add documentation for eia608_extract parameter

Author: MarcAntoine-Arnaud

doc/demuxers.texi

@@ -776,6 +776,19 @@ MJPEG stream. Turning this option on by setting it to 1 will result in a stricte
 of the boundary value.
 @end table
 
+@section mxf
+
+MXF demuxer.
+
+@table @option
+
+@item -eia608_extract @var{bool}
+Extract EIA-608 captions from SMPTE 436M track
+
+@item -skip_audio_reordering @var{bool}
+This option will disable the audio reordering based on Multi-Channel Audio (MCA) labelling (SMPTE ST-377-4).
+@end table
+
 @section rawvideo
 
 Raw video demuxer.