deprecate the phpcr_odm_reference_collection form type in favor of phpcr_document

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | yes
| Deprecations? | yes
| Tests pass?   | yes
| Fixed tickets | relates to https://github.com/symfony-cmf/symfony-cmf-docs/pull/347
| License       | MIT
| Doc PR        | https://github.com/symfony-cmf/symfony-cmf-docs/pull/351

afaict this form type does not even work (anymore?) and it seems the only thing it does that phpcr_document can not do better already is to allow to use the UUID instead of the path (as path is the primary id for phpcr-odm). not sure if that is worth anything - but if so we should port that into the phpcr_document type.

if we deprecate this now, would we drop it in version 1.2? i *think* this is not even usable, but i might be wrong so we could use the deprecation to hear from users.