update collection.rst - add cascade_validation opt

| Q             | A
| ------------- | ---
| Doc fix?      | no
| New docs?     | no
| Applies to    | all

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".