group hit-test flags into an enum (like I did in docs); this makes easier to document...

group hit-test flags into an enum (like I did in docs); this makes easier to document them, but is also logically the best thing since wxRichTextBuffer::HitTest is documented to return only _one_ of those flags.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@56219 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775