Improve EVT_SHOW documentation.

Mention that it applies to the TLWs only and explain when it is generated (and
not generated).

Closes #12353.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@65316 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

interface/wx/toplevel.h

@@ -59,6 +59,8 @@ enum
         Process a @c wxEVT_MOVE_END event, which is generated when the user stops
         moving or sizing a window. wxMSW only.
         See wxMoveEvent.
+    @event{EVT_SHOW(func)}
+        Process a @c wxEVT_SHOW event. See wxShowEvent.
     @endEventTable
 
     @library{wxcore}