Document surprising behaviour of wxDatePickerCtrl::GetRange() in wxMSW.

wxMSW native control always has a built-in lower limit as MSW doesn't support
the dates before the year 1601 so wxDatePickerCtrl::GetRange() always returns
a valid range start, even if SetRange() hadn't been called to set it.

As this can be unexpected, document this behaviour clearly.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@67988 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

interface/wx/datectrl.h

@@ -105,6 +105,11 @@ public:
         range is set (or only one of the bounds is set), @a dt1 and/or @a dt2
         are set to be invalid.
 
+        Notice that when using a native MSW implementation of this control the
+        lower range is always set, even if SetRange() hadn't been called
+        explicitly, as the native control only supports dates later than year
+        1601.
+
         @param dt1
             Pointer to the object which receives the lower range limit or
             becomes invalid if it is not set. May be @NULL if the caller is not