mention that SetItemToolTip() is for wxMSW and wxGTK2 only

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@39040 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

docs/latex/wx/radiobox.tex

@@ -345,6 +345,9 @@ a wxEVT\_COMMAND\_RADIOBOX\_SELECTED event to get emitted.
 
 Sets the tooltip text for the specified item in the radio group.
 
+{\bf Platform note:} Currently only implemented in wxMSW and wxGTK2 and does
+nothing in the other ports.
+
 \wxheading{Parameters}
 
 \docparam{item}{Index of the item the tooltip will be shown for.}