Document wxWindow::FindWindow() TLW skipping behaviour.

Explicitly mention that TLW "children" are not searched by this function
(which is the case since r74721).

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@75608 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775