Document wxWindow::HitTest().

This is not a very useful method, but by providing at least some documentation
for it, we prevent it from inheriting the docs for the next method, which have
nothing to do with it.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@77751 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775