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.

tests/controls/datepickerctrltest.cpp

@@ -106,6 +106,13 @@ void DatePickerCtrlTestCase::Range()
 
     m_datepicker->SetValue(dtStart - wxTimeSpan::Day());
     CPPUNIT_ASSERT_EQUAL( dtEnd, m_datepicker->GetValue() );
+
+
+    // Changing the range should clamp the current value to it if necessary.
+    const wxDateTime
+        dtBeforeEnd = dtEnd - wxDateSpan::Day();
+    m_datepicker->SetRange(dtStart, dtBeforeEnd);
+    CPPUNIT_ASSERT_EQUAL( dtBeforeEnd, m_datepicker->GetValue() );
 }
 
 #endif // wxUSE_DATEPICKCTRL