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.

docs/changes.txt

@@ -96,6 +96,9 @@ Changes in behaviour not resulting in compilation errors
 - wxAuiNotebook::RemovePage() now hides the removed page, so it needs to be
   shown again if it is reused in another place.
 
+- wxSizer::RecalcSizes() shouldn't be called directly (note that it was never
+  supposed to be called, only implemented), call Layout() instead.
+
 
 Changes in behaviour which may result in build errors
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