minor symfony#6464 Added possible values for access_decision_manager.strategy (AAstakhov)

This PR was merged into the 2.3 branch.

Discussion
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Added possible values for access_decision_manager.strategy

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | no
| Applies to    | 2.3+
| Fixed tickets | n/a

Added a comment with the list of possible values for access_decision_manager.strategy (like for session_fixation_strategy)

Commits
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73d96d4 Added possible values for access_decision_manager.strategy

Author: weaverryan

reference/configuration/security.rst

@@ -27,6 +27,7 @@ Each part will be explained in the next section.
             always_authenticate_before_granting:  false
             erase_credentials:    true
             access_decision_manager:
+                # strategy can be: affirmative, consensus, unanimous
                 strategy:             affirmative
                 allow_if_all_abstain:  false
                 allow_if_equal_granted_denied:  true