Document and test wxDatePickerCtrl::SetRange() date-clamping behaviour.

Document that the current value of the control is adjusted if it doesn't lie
in the newly set range and add a unit test to check for this, now that it
actually passes under all platforms, after the previous commits.

interface/wx/datectrl.h

@@ -173,6 +173,10 @@ public:
         becomes the earliest date (inclusive) accepted by the control. If
         @a dt2 is valid, it becomes the latest possible date.
 
+        Notice that if the current value is not inside the new range, it will
+        be adjusted to lie inside it, i.e. calling this method can change the
+        control value, however no events are generated by it.
+
         @remarks If the current value of the control is outside of the newly
                  set range bounds, the behaviour is undefined.
     */