minor #16944 [Security][Guard] checkCredentials() should have a @return annotation (derrabus)

This PR was merged into the 2.8 branch.

Discussion
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[Security][Guard] checkCredentials() should have a @return annotation

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | none
| License       | MIT
| Doc PR        | none

According to its documentation, an implementation of the `GuardAuthenticatorInterface::checkCredentials()` method should return `true` on success. I've added a `@return` annotation to the PHPDoc block to reflect the fact that this method should actually return something.

Commits
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4fd24a0 Added @return to checkCredentials()

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

@@ -83,6 +83,8 @@ interface GuardAuthenticatorInterface extends AuthenticationEntryPointInterface
      *
      * @param mixed         $credentials
      * @param UserInterface $user
+     * 
+     * @return bool
      *
      * @throws AuthenticationException
      */