Add selection gradient doc placeholder

doc/lua.adoc

@@ -646,6 +646,14 @@ Example:
 
  local value = selection.get(sel, x, y);
 
+=== gradient
+
+Create a "gradient" of selected positions.
+
+Example:
+
+ local s = selection.gradient({ type = "radial", x = 3, y = 5, x2 = 10, y2 = 12, mindist = 4, maxdist = 10, limited = false });
+
 === grow
 
 Add locations to the selection by choosing unselected locations