Fixed description of session storage of the ApiKeyAuthenticator

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | no
| Applies to    | 2.4
| Fixed tickets | #4060

I assume the authentication is needed for each request (even if token is stored in the session)
since you can add custom logic in the authenticator.
Remove this here - the `::` in the paragraph above (the key is that there are 2 colons) implies the `.. code-block:: php` (only works with PHP and is special to our setup). That's the reason for the Travis failure :).