Don't pretend that wxAnyButton::SetBitmap() supports invalid bitmaps.

Because it doesn't under MSW.

See #13569.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@74430 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

interface/wx/anybutton.h

@@ -103,8 +103,9 @@ public:
         states.
 
         @param bitmap
-            The bitmap to display in the button. May be invalid to remove any
-            currently displayed bitmap.
+            The bitmap to display in the button. Under wxGTK the bitmap can be
+            invalid to remove the currently displayed bitmap, however this is
+            not supported in wxMSW currently.
         @param dir
             The position of the bitmap inside the button. By default it is
             positioned to the left of the text, near to the left button border.