warn when backref will replace existing userland descriptor

warn when backref will replace existing userland descriptor

A warning is emitted if a backref name used in :func:`_orm.relationship`
names an attribute on the target class which already has a method or
attribute assigned to that name, as the backref declaration will replace
that attribute.

Fixes: #4629
Change-Id: I0059b35ce60f43b0f3d8be008f12411154484ea1