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

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