[Security] Use the `is_granted` expression to keep consistency with php example above

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | no
| Applies to    | 2.4
| Fixed tickets | - 

As `has_role` expression only check for roles (simple `in_array`) without calling the `AccessDecisionManager`, it does not do the same job that
`$this->get('security.context')->isGranted('ROLE_ADMIN')` used in the php example just above.

It is quiete confusing.
I would add something like "(this can ony be used in the ``@Security`` annotation)" at the end of this item, to warn users and avoid confusion.