Discourage the use of wxTAB_TRAVERSAL in the documentation

This style should almost never be used in the application code, it's mostly an
implementation detail (but not quite as it might make sense to turn it off for
some wxPanel, at least in theory).

interface/wx/window.h

@@ -150,7 +150,9 @@ enum wxWindowVariant
            The window is transparent, that is, it will not receive paint
            events. Windows only.
     @style{wxTAB_TRAVERSAL}
-           Use this to enable tab traversal for non-dialog windows.
+           This style is used by wxWidgets for the windows supporting TAB
+           navigation among their children, such as wxDialog and wxPanel. It
+           should almost never be used in the application code.
     @style{wxWANTS_CHARS}
            Use this to indicate that the window wants to get all char/key
            events for all keys - even for keys like TAB or ENTER which are