avfilter/af_surround: add support for some upmixing of 3.0, 2.1 and 5.1 channel layout

Signed-off-by: Paul B Mahol <onemda@gmail.com>

doc/filters.texi

@@ -3795,7 +3795,7 @@ Set level of input signal of original channel. Default is 0.8.
 @section surround
 Apply audio surround upmix filter.
 
-This filter allows to produce multichannel output from stereo audio stream.
+This filter allows to produce multichannel output from audio stream.
 
 The filter accepts the following options:
 
@@ -3806,6 +3806,12 @@ Set output channel layout. By default, this is @var{5.1}.
 See @ref{channel layout syntax,,the Channel Layout section in the ffmpeg-utils(1) manual,ffmpeg-utils}
 for the required syntax.
 
+@item chl_in
+Set input channel layout. By default, this is @var{stereo}.
+
+See @ref{channel layout syntax,,the Channel Layout section in the ffmpeg-utils(1) manual,ffmpeg-utils}
+for the required syntax.
+
 @item level_in
 Set input volume level. By default, this is @var{1}.