disallow adding to identity map that's been discarded

disallow adding to identity map that's been discarded

Fixed bug where iterating a :class:`.Result` from a :class:`_orm.Session`
after that :class:`_orm.Session` were closed would partially attach objects
to that session in an essentially invalid state.   It now raises an exception
with a link to new documentation if an **un-buffered** result is iterated
from a :class:`_orm.Session` that was closed or otherwise had the
:meth:`_orm.Session.expunge_all` method called after that :class:`.Result`
was generated.  The "prebuffer_rows" execution option, as is used by the
asyncio extension, may be used to produce a :class:`.Result` where the ORM
objects are prebuffered, and in this case iterating the result will produce
a series of detached objects.

Fixes: #7128
Change-Id: I59f0ae32a83a64587937741b80f31ff825bbb574