Document incompatible change to wxWindow::FindWindow() in 3.0.

We now skip the TLW children in this method, see r74721.

Also explicitly mention that TLW "children" are not searched by this function
in the documentation.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/branches/WX_3_0_BRANCH@75607 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775