Document wxSizer::RecalcSizes() and RepositionChildren() better

Mention that RecalcSizes() shouldn't be called any more, after the
changes of 622deec262 (Replace wxSizer::RecalcSizes() with
RepositionChildren(), 2015-10-11), and that RepositionChildren()
replacing it shouldn't be directly called neither.

interface/wx/sizer.h

@@ -733,6 +733,10 @@ public:
         total size available to the sizer (@c m_size) and the size computed by
         the last call to CalcMin().
 
+        Note that you should never call this method directly, call Layout()
+        instead if you need to manually update the sizer elements positions.
+        This method is only called by wxWidgets itself.
+
         @since 3.1.3, before this version RecalcSizes() method not taking any
             arguments had to be overridden in the derived classes instead.
     */