doc/muxers: remove outdated comment about live flags for the segment muxer

The live flags is currently unused, but still kept for backward
compatibility reasons (and in case it turns to be useful again for
whatever reason).

doc/muxers.texi

@@ -538,10 +538,6 @@ Allow caching (only affects M3U8 list files).
 
 @item live
 Allow live-friendly file generation.
-
-This currently only affects M3U8 lists. In particular, write a fake
-EXT-X-TARGETDURATION duration field at the top of the file, based on
-the specified @var{segment_time}.
 @end table
 
 Default value is @code{cache}.