Rewrite the "deleting from collections" section

This section documents an aspect of the ORM that is
more generalized than just collections as well as more general
than just using session.delete(), even though session.delete()
with collections is the most common use case.   Try to expand
out the scope here to explain the situation from the perspective
of any kind of relationship, include example with delete cascade
with many-to-one as well.

Change-Id: Ifacb91f90951d1b0da6dbadc2d72273d79b756e2
Fixes: #4191

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