Add examples in documentation showing how to avoid to decode audio and

output video in pass 1 for 2-pass encoding.
Patch by Erwan Ducroquet erwan _dot_ ducroquet _at_ gmail _dot_ com

Originally committed as revision 14736 to svn://svn.ffmpeg.org/ffmpeg/trunk

doc/ffmpeg-doc.texi

@@ -429,6 +429,12 @@ Select the pass number (1 or 2). It is useful to do two pass
 encoding. The statistics of the video are recorded in the first
 pass and the video is generated at the exact requested bitrate
 in the second pass.
+On pass 1, you may just deactivate audio and set output to null,
+examples for Windows and Unix:
+@example
+ffmpeg -i foo.mov -vcodec libxvid -pass 1 -an -f rawvideo -y NUL
+ffmpeg -i foo.mov -vcodec libxvid -pass 1 -an -f rawvideo -y /dev/null
+@end example
 
 @item -passlogfile @var{file}
 Set two pass logfile name to @var{file}.