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 outdated reference to wxPanel always calling Layout() and mention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

docs/doxygen/overviews/windowsizing.h

@@ -116,4 +116,9 @@ some simple explanations of things.
     the constraints algorithm is run. The @c Layout() method is what is called by
     the default @c EVT_SIZE handler for container windows.
 
+@li wxTopLevelWindow::Layout(): this overridden version does the same thing as
+    the base wxWindow::Layout() except, for convenience, it will also resize
+    the only child of the top-level window to cover its entire client area if
+    there is no sizer associated with the window. Note that this only happens
+    if there is exactly one child.
 */

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();
 

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();