Mention that pages can't be added to wxBookCtrl more than once.

Apparently some people expect this to work although it's really not meant to.

See #13209.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@68345 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

interface/wx/bookctrl.h

@@ -221,6 +221,10 @@ public:
 
     /**
         Adds a new page.
+
+        The page must have the book control itself as the parent and must not
+        have been added to this control previously.
+
         The call to this function may generate the page changing events.
 
         @param page