doc: add a few links to the Channel Layout syntax section

doc/filters.texi

@@ -541,6 +541,8 @@ A '|'-separated list of requested sample rates.
 @item channel_layouts
 A '|'-separated list of requested channel layouts.
 
+See @ref{channel layout syntax,,the Channel Layout section in the ffmpeg-utils(1) manual,ffmpeg-utils}
+for the required syntax.
 @end table
 
 If a parameter is omitted, all values are allowed.

doc/resampler.texi

@@ -42,10 +42,11 @@ Set the internal sample format. Default value is @code{none}.
 This will automatically be chosen when it is not explicitly set.
 
 @item icl, in_channel_layout
-Set the input channel layout.
-
 @item ocl, out_channel_layout
-Set the output channel layout.
+Set the input/output channel layout.
+
+See @ref{channel layout syntax,,the Channel Layout section in the ffmpeg-utils(1) manual,ffmpeg-utils}
+for the required syntax.
 
 @item clev, center_mix_level
 Set the center mix level. It is a value expressed in deciBel, and must be