Improve delete cascade and passive deletes sections

Improve delete cascade and passive deletes sections

Add cross linking from Core to ORM and also add new sections
and examples regarding many-to-many.    Move the section
out of "collections" and into the "cascades" chapter
where it's more likely to be found.

Change-Id: If29360e463e5745279bc5335bd12352d50ad8953
(cherry picked from commit 9e1ee412b8650761af6df993e119906682604728)