Fix up API docs.
Stop fatal errors when _joinData is present and not an entity.

If _joinData is set to an array property it should not cause failures
when attempting to save. Array values should be converted into entities
elsewhere in the process.

Refs #5130
Typehint against interface not implementation.

Use the EntityInterface for typehinting and not Entity itself.
Fix doc block errors and CS error.
Merge pull request #5131 from cakephp/issue-5130

Fix fatal errors on _joinData