minor #23732 [2.8][Security][Guard] Docblock improvement (vudaltsov)

This PR was merged into the 2.8 branch.

Discussion
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[2.8][Security][Guard] Docblock improvement

| Q             | A
| ------------- | ---
| Branch?       | 2.8
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | n/a
| License       | MIT
| Doc PR        | n/a

Added this small detail which might prevent [exception](https://github.com/symfony/symfony/blob/2.8/src/Symfony/Component/Security/Guard/Firewall/GuardAuthenticationListener.php#L193) when developing

Commits
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ce86449 Docblock improvement

src/Symfony/Component/Security/Guard/GuardAuthenticatorInterface.php

@@ -153,6 +153,7 @@ interface GuardAuthenticatorInterface extends AuthenticationEntryPointInterface
      *      done by having a _remember_me checkbox in your form, but
      *      can be configured by the "always_remember_me" and "remember_me_parameter"
      *      parameters under the "remember_me" firewall key
+     *  D) The onAuthenticationSuccess method returns a Response object
      *
      * @return bool
      */