Emphasize that wxDC::Clear() used brush set by SetBackground()

Ensure that people don't get the impression that using SetBrush() is
supposed to affect Clears().

See #18463.

interface/wx/dc.h

@@ -257,6 +257,10 @@ public:
 
     /**
         Clears the device context using the current background brush.
+
+        Note that SetBackground() method must be used to set the brush used by
+        Clear(), the brush used for filling the shapes set by SetBrush() is
+        ignored by it.
     */
     void Clear();