avfilter/af_surround: make channel spread from stereo image user configurable

doc/filters.texi

@@ -4889,6 +4889,18 @@ Set LFE input volume. By default, this is @var{1}.
 @item lfe_out
 Set LFE output volume. By default, this is @var{1}.
 
+@item allx
+Set spread usage of stereo image across X axis for all channels.
+
+@item ally
+Set spread usage of stereo image across Y axis for all channels.
+
+@item fcx, flx, frx, blx, brx, slx, srx, bcx
+Set spread usage of stereo image across X axis for each channel.
+
+@item fcy, fly, fry, bly, bry, sly, sry, bcy
+Set spread usage of stereo image across Y axis for each channel.
+
 @item win_func
 Set window function.