Recommend using wxStaticBox as parent more strongly

Not doing it, i.e. creating the window inside the static box as its
siblings, rather than children, may appear to work, but actually doesn't
always do it, for example the windows are not shown correctly when using
RTL locale.

Closes #19086.

interface/wx/sizer.h

@@ -1892,7 +1892,7 @@ public:
     itself as a convenience. In any case, the sizer owns the wxStaticBox control
     and will delete it in the wxStaticBoxSizer destructor.
 
-    Note that since wxWidgets 2.9.1 you are encouraged to create the windows
+    Note that since wxWidgets 2.9.1 you are strongly encouraged to create the windows
     which are added to wxStaticBoxSizer as children of wxStaticBox itself, see
     this class documentation for more details.