ffmpeg source update for sep 09

source/patched-ffmpeg-mt/doc/ffmpeg-doc.texi

Index: source/patched-ffmpeg-mt/doc/ffmpeg-doc.texi
===================================================================
--- source/patched-ffmpeg-mt/doc/ffmpeg-doc.texi	(revision 59334)
+++ source/patched-ffmpeg-mt/doc/ffmpeg-doc.texi	(working copy)
@@ -510,7 +510,10 @@
 @item -aq @var{q}
 Set the audio quality (codec-specific, VBR).
 @item -ac @var{channels}
-Set the number of audio channels (default = 1).
+Set the number of audio channels. For input streams it is set by
+default to 1, for output streams it is set by default to the same
+number of audio channels in input. If the input file has audio streams
+with different channel count, the behaviour is undefined.
 @item -an
 Disable audio recording.
 @item -acodec @var{codec}