Document unexpected default wxGraphicsContext interpolation quality.

In 3.0, keep wxINTERPOLATION_GOOD as the default value instead of
wxINTERPOLATION_DEFAULT for compatibility, but at least mention this and warn
about the changes in 3.1.

See #14134.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/branches/WX_3_0_BRANCH@76304 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

interface/wx/graphics.h

@@ -810,6 +810,14 @@ public:
         Sets the interpolation quality, returns true if it is supported.
 
         Not implemented in Cairo backend currently.
+
+        Notice that in wxWidgets 3.0 the default interpolation quality for
+        GDI+-based implementation is ::wxINTERPOLATION_GOOD and @e not
+        ::wxINTERPOLATION_DEFAULT (unlike under OS X with CoreGraphics-based
+        implementation). This will be changed in wxWidgets 3.1 and later
+        version, call this method explicitly instead of relying on the default
+        value to ensure consistent behaviour across different platforms and
+        versions.
      */
     virtual bool SetInterpolationQuality(wxInterpolationQuality interpolation) = 0;