Fix Fl_Window::size_range() documentation (STR 3352)

"Tiny window problem if child group larger than window"

See Greg's comment 4: "something is wrong here: either the logic or
the docs are incorrect, not sure which."

It turned out that both were incorrect.

Since we fixed the code this commit completes the STR by fixing the
docs as well so they match the behavior.

FL/Fl_Group.H

@@ -162,6 +162,10 @@ public:
 
     See the \ref resize chapter for more examples and detailed explanation.
 
+    \note The resizable() widget of a window can also affect the window's
+      resizing behavior if Fl_Window::size_range() is not called.
+
+    \see Fl_Window::size_range()
   */
   void resizable(Fl_Widget* o) {resizable_ = o;}
   /**