Document possible problem with unbinding functors.

Unbind() may currently disconnect the wrong functor as it compares them by
address.

See http://thread.gmane.org/gmane.comp.lib.wxwidgets.general/81445

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@76033 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775