Rename _delete to _destroy in nested attributes

When nested attributes was released in 2.3 it came with a small inconsistency. The DSL uses :allow_destroy, the method called in the associated object is :destroy but the trigger is called _delete:

  accepts_nested_attributes_for :tasks, :allow_destroy => true

This patch changes _delete to _destroy but adds a deprecation warning. Documentation was also updated.

As already discussed in the mailing list, someone wanted to add a _delete behavior that will actually delete the associated record. So renaming _delete to _destroy now makes room for improvements in the future.