[WIP] add notes on collection and entity field types

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | yes
| Applies to    | all
| Fixed tickets | #1566

I added notes at the top of the collection and entity field types, directing the user to more specific documentation (and the cookbook article) if they are working with collections of Doctrine entities. Hopefully, this will point people in the right direction.