doc/filters: indicate commands for zscale

doc/filters.texi

@@ -20445,7 +20445,17 @@ horizontal and vertical output chroma subsample values. For example for the
 pixel format "yuv422p" @var{hsub} is 2 and @var{vsub} is 1.
 @end table
 
+@subsection Commands
+
+This filter supports the following commands:
 @table @option
+@item width, w
+@item height, h
+Set the output video dimension expression.
+The command accepts the same syntax of the corresponding option.
+
+If the specified expression is not valid, it is kept at its current
+value.
 @end table
 
 @c man end VIDEO FILTERS