- in depth docs about some merge() tips

- in depth docs about some merge() tips
- docs about backref cascade
- Another new flag on relationship(), cascade_backrefs,
disables the "save-update" cascade when the event was
initiated on the "reverse" side of a bidirectional
relationship.   This is a cleaner behavior so that
many-to-ones can be set on a transient object without
it getting sucked into the child object's session,
while still allowing the forward collection to
cascade.   We *might* default this to False in 0.7.