doc/filters: document amplify commands

doc/filters.texi

@@ -6183,6 +6183,18 @@ This option controls maximum possible value that will increase source pixel valu
 Set which planes to filter. Default is all. Allowed range is from 0 to 15.
 @end table
 
+@subsection Commands
+
+This filter supports the following @ref{commands} that corresponds to option of same name:
+@table @option
+@item factor
+@item threshold
+@item tolerance
+@item low
+@item high
+@item planes
+@end table
+
 @section ass
 
 Same as the @ref{subtitles} filter, except that it doesn't require libavcodec