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 outdated reference to wxPanel always calling Layout() and mention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/window.h

@@ -3445,15 +3445,20 @@ public:
     void SetConstraints(wxLayoutConstraints* constraints);
 
     /**
-        Invokes the constraint-based layout algorithm or the sizer-based algorithm
-        for this window.
+        Lays out the children of this window using the associated sizer.
 
-        This function does not get called automatically when the window is resized
-        because lots of windows deriving from wxWindow does not need this functionality.
-        If you want to have Layout() called automatically, you should derive
-        from wxPanel (see wxPanel::Layout).
+        If a sizer hadn't been associated with this window (see SetSizer()),
+        this function doesn't do anything, unless this is a top level window
+        (see wxTopLevelWindow::Layout()).
+
+        Note that this method is called automatically when the window size
+        changes if it has the associated sizer (or if SetAutoLayout() with
+        @true argument had been explicitly called), ensuring that it is always
+        laid out correctly.
 
         @see @ref overview_windowsizing
+
+        @returns Always returns @true, the return value is not useful.
     */
     virtual bool Layout();