Document wxFileConfig ctor.

This ctor was not documented at all because the comment preceding it was a
plain C++ comment and not a Doxygen one.

Do it now, especially as the logic of this ctor is not quite obvious.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@69976 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775