Document CreateMeasuringContext now that it is available for all wxGC impls.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@60246 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

interface/wx/graphics.h

@@ -700,6 +700,12 @@ public:
     */
     virtual wxGraphicsContext* CreateContextFromNativeWindow(void* window) = 0;
 
+    /**
+        Creates a wxGraphicsContext that can be used for measuring texts only. 
+        No drawing commands are allowed.
+    */
+    virtual wxGraphicsContext * CreateMeasuringContext() = 0;
+
     /**
         Creates a native graphics font from a wxFont and a text colour.
     */