doc/ffmpeg: Clarify that the sdp_file option requires an rtp output.

Signed-off-by: NSimon Thelen <ffmpeg-dev@c-14.de>
Signed-off-by: NMichael Niedermayer <michael@niedermayer.cc>

doc/ffmpeg.texi

@@ -1233,9 +1233,9 @@ 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}.
+Print sdp information for an output stream to @var{file}.
 This allows dumping sdp information when at least one output isn't an
-rtp stream.
+rtp stream. (Requires at least one of the output formats to be rtp).
 
 @item -discard (@emph{input})
 Allows discarding specific streams or frames of streams at the demuxer.