docs/ffserver: document ffm/ffm2

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

doc/ffserver.texi

@@ -238,6 +238,19 @@ You use this by adding the ?date= to the end of the URL for the stream.
 For example:   @samp{http://localhost:8080/test.asf?date=2002-07-26T23:05:00}.
 @c man end
 
+@section What is FFM, FFM2
+
+FFM and FFM2 are formats used by ffserver, they allow storing a wide varity of
+video and audio streams, encoding options and can store a moving time segment
+of a infinite movie or a whole movie.
+
+FFM, is version specific, there is limited compatibility of FFM files
+generated by one version of ffmpeg/ffserver and another version of
+ffmpeg/ffserver. It may work but its not guranteed to work.
+
+FFM2 is extensible while maintaining compatibility and should work between
+differing versions of tools. FFM2 is the default.
+
 @chapter Options
 @c man begin OPTIONS