Extract private methods when generating documentation too.

The only private methods appearing in interface/wx/*.h files are meant to be
documented, so do extract them. This should fix wxThread::OnExit() not
appearing in the generated documentation, for example.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@75858 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775