add docs for the #define of wxBookCtrl

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@56271 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

interface/wx/bookctrl.h

@@ -336,6 +336,13 @@ public:
 */
 };
 
+/**
+    wxBookCtrl is defined to one of the 'real' book controls.
+
+    See @ref overview_bookctrl for more info.
+*/
+#define wxBookCtrl      TheBestBookCtrlForTheCurrentPlatform
+
 
 /**
     @class wxBookCtrlEvent