[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.
I would like to keep the reference to the multiple entity managers chapter:

```rst
If you use multiple entity managers (for example when using
:doc:`use multiple entity managers </cookbook/doctrine/multiple_entity_managers>`),
you can specify which manager
to use with the manager_name option:
```

What do you think?