Update wxWindow and wxTopLevelWindow::Layout() documentation

Explain how these methods actually work, remove a very (from wx 1.x
days?) outdated reference to wxPanel always calling Layout() and mention
the special case of wxTopLevelWindow in the overview too.

interface/wx/toplevel.h

@@ -283,8 +283,20 @@ public:
     bool IsUsingNativeDecorations() const;
 
     /**
-        See wxWindow::SetAutoLayout(): when auto layout is on, this function gets
-        called automatically when the window is resized.
+        Lays out the children using the window sizer or resizes the only child
+        of the window to cover its entire area.
+
+        This class overrides the base class Layout() method to check if this
+        window contains exactly one child -- which is commonly the case, with
+        wxPanel being often created as the only child of wxTopLevelWindow --
+        and, if this is the case, resizes this child window to cover the entire
+        client area.
+
+        Note that if you associate a sizer with this window, the sizer takes
+        precedence and the only-child-resizing is only used as fallback.
+
+        @returns @false if nothing was done because the window doesn't have
+                 neither a sizer nor a single child, @true otherwise.
     */
     virtual bool Layout();