faq: document af_aconvert quirks using lavfi.

doc/faq.texi

@@ -292,6 +292,24 @@ The solution is to apply the @option{-profile} option to the video stream only
 by using @url{http://ffmpeg.org/ffmpeg.html#Stream-specifiers-1, Stream specifiers}.
 Appending @code{:v} to it will do exactly that.
 
+@section Using @option{-f lavfi}, audio becomes mono for no apparent reason.
+
+Use @option{-dumpgraph -} to find out exactly where the channel layout is
+lost.
+
+Most likely, it is through @code{auto-inserted aconvert}. Try to understand
+why the converting filter was needed at that place.
+
+Just before the output is a lilely place, as @option{-f lavfi} currently
+only support packed S16.
+
+Then insert the correct @code{aconvert} explicitly in the filter graph,
+specifying the exact format.
+
+@example
+aconvert=s16:stereo:packed
+@end example
+
 @chapter Development
 
 @section Are there examples illustrating how to use the FFmpeg libraries, particularly libavcodec and libavformat?