Document wxDocument::DeleteContents().

Document wxDocument::DeleteContents().

This method purpose was not quite clear as it was only mentioned in passing in
documentation of OnCloseDocument() but not really documented itself.

Also don't call it from wxDocument dtor as this is useless: the user-defined
overridden version will never be called from here and the base class version
does nothing.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@64299 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775