Custom classes should typically override DoGetBestClientSize() instead of
DoGetBestSize() to avoid having to deal with the borders.
Also don't reference the deprecated SetInitialBestSize() unnecessarily and
document its non-deprecated replacement.
git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@71391 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775