[Cookbook][Security] usage of a non-default entity manager in an entity user provider

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | no
| Applies to    | all
| Fixed tickets | #4021

The cookbook on entity user providers for the Security component only
describes how to create an entity provider using the default entity
manager. This may not be sufficient enough if you have multiple entity
managers and therefore be eplicit with the entity manager to use in
the user provider.
Love this note! Do you think we can shorten it a little?

>
If you use multiple entity managers, you can specify which manager
to use with the ``manager_name`` option:

What do you think?