minor #19773 [Security] Added note inside phpdoc. (Koc)

This PR was squashed before being merged into the 2.8 branch (closes #19773).

Discussion
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[Security] Added note inside phpdoc.

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

Commits
-------

c49e462 [Security] Added note inside phpdoc.

src/Symfony/Component/Security/Core/Authorization/Voter/Voter.php

@@ -74,6 +74,7 @@ abstract class Voter implements VoterInterface
 
     /**
      * Perform a single access check operation on a given attribute, subject and token.
+     * It is safe to assume that $attribute and $subject already passed the "supports()" method check.
      *
      * @param string         $attribute
      * @param mixed          $subject