Document wxListCtrl::InReportView() and IsVirtual().

These functions are more convenient than directly testing for styles and are
already used outside wxWidgets do make them officially part of the public API.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@70075 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775