doc: document amerge filter as an alternative for the -map_channel limitation.

doc/ffmpeg.texi

@@ -835,6 +835,18 @@ possible, for example, to turn two separate mono streams into a single stereo
 stream. However spliting a stereo stream into two single channel mono streams
 is possible.
 
+If you need this feature, a possible workaround is to use the @emph{amerge}
+filter. For example, if you need to merge a media (here @file{input.mkv}) with 2
+mono audio streams into one single stereo channel audio stream (and keep the
+video stream):
+
+@example
+ffmpeg -i input.mkv -f lavfi -i "
+amovie=input.mkv:si=1 [a1];
+amovie=input.mkv:si=2 [a2];
+[a1][a2] amerge" -c:a pcm_s16le -c:v copy output.mkv
+@end example
+
 @item -map_metadata[:@var{metadata_spec_out}] @var{infile}[:@var{metadata_spec_in}] (@emph{output,per-metadata})
 Set metadata information of the next output file from @var{infile}. Note that
 those are file indices (zero-based), not filenames.

doc/filters.texi

@@ -189,6 +189,23 @@ Example: merge two mono files into a stereo stream:
 amovie=left.wav [l] ; amovie=right.mp3 [r] ; [l] [r] amerge
 @end example
 
+If you need to do multiple merges (for instance multiple mono audio streams in
+a single video media), you can do:
+@example
+ffmpeg -f lavfi -i "
+amovie=input.mkv:si=0 [a0];
+amovie=input.mkv:si=1 [a1];
+amovie=input.mkv:si=2 [a2];
+amovie=input.mkv:si=3 [a3];
+amovie=input.mkv:si=4 [a4];
+amovie=input.mkv:si=5 [a5];
+[a0][a1] amerge [x0];
+[x0][a2] amerge [x1];
+[x1][a3] amerge [x2];
+[x2][a4] amerge [x3];
+[x3][a5] amerge" -c:a pcm_s16le output.mkv
+@end example
+
 @section anull
 
 Pass the audio source unchanged to the output.