minor #6130 [Security][Guard] Completed start method signature (jeremyFreeAgent)

This PR was submitted for the master branch but it was merged into the 2.8 branch instead (closes #6130).

Discussion
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[Security][Guard] Completed start method signature

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | no
| Applies to    | 2.8
| Fixed tickets |

The ``start()`` method did not have its signature but the other methods did.

Commits
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b6b6a39 [Security][Guard] Completed start method signature

Author: xabbuh

cookbook/security/guard-authentication.rst

@@ -418,7 +418,7 @@ Each authenticator needs the following methods:
     object that should be sent to the client. The ``$exception`` will tell you
     *what* went wrong during authentication.
 
-**start**
+**start(Request $request, AuthenticationException $authException = null)**
     This is called if the client accesses a URI/resource that requires authentication,
     but no authentication details were sent (i.e. you returned ``null`` from
     ``getCredentials()``). Your job is to return a