update collection.rst - add cascade_validation opt

It makes sense to describe cascade_validation option in collection
documentation. `Valid` constraint does not suit all needs, especially
when we have to set validation_groups for embed object/forms, and so
cascade_validation should not be that "hidden".
minor #3538 Rebasing #3518 (weaverryan)

This PR was merged into the 2.3 branch.

Discussion
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Rebasing #3518

This is just a rebase of #3518 against the 2.3 branch so that it can be merged cleanly.

Commits
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561d201 update collection.rst - add cascade_validation opt