Document that wxScreenDC corresponds to the entire virtual screen

And not just the primary monitor.

See #13279.

interface/wx/dcscreen.h

@@ -11,6 +11,10 @@
     A wxScreenDC can be used to paint on the screen. This should normally be
     constructed as a temporary stack object; don't store a wxScreenDC object.
 
+    When using multiple monitors, wxScreenDC corresponds to the entire virtual
+    screen composed of all of them. Notice that coordinates on wxScreenDC can
+    be negative in this case, see wxDisplay::GetGeometry() for more.
+
     @library{wxcore}
     @category{dc}