Make "NOT the index" in wxListCtrl::SortItems() stand out even more.

Any reasonable person expects the sort callback to receive the indices of the
items, but it doesn't: it's passed the client data associated with them
instead. Make it even more noticeable in the documentation.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@76205 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775