doc/ffmpeg.texi: document the new -sdp_file option

Signed-off-by: Simon Thelen <ffmpeg-dev@c-14.de>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

doc/ffmpeg.texi

@@ -1160,6 +1160,11 @@ requested by @command{ffserver}.
 The option is intended for cases where features are needed that cannot be
 specified to @command{ffserver} but can be to @command{ffmpeg}.
 
+@item -sdp_file @var{file} (@emph{global})
+Print sdp information to @var{file}.
+This allows dumping sdp information when at least one output isn't an
+rtp stream.
+
 @item -discard (@emph{input})
 Allows discarding specific streams or frames of streams at the demuxer.
 Not all demuxers support this.