Mention that wxDisplaySize() shouldn't be used in the new code

Prefer to use wxDisplay::GetGeometry() which works for any display, not
just the primary one.

interface/wx/gdicmn.h

@@ -1262,6 +1262,10 @@ wxSize wxGetDisplayPPI();
 /**
     Returns the display size in pixels.
 
+    @note Use of this function is not recommended in the new code as it only
+        works for the primary display. Use wxDisplay::GetGeometry() to retrieve
+        the size of the appropriate display instead.
+
     Either of output pointers can be @NULL if the caller is not interested in
     the corresponding value.
 
@@ -1277,6 +1281,10 @@ void wxDisplaySize(int* width, int* height);
 /**
     Returns the display size in pixels.
 
+    @note Use of this function is not recommended in the new code as it only
+        works for the primary display. Use wxDisplay::GetGeometry() to retrieve
+        the size of the appropriate display instead.
+
     @see wxDisplay
 
     @header{wx/gdicmn.h}