minor #9125 Add missing allow_if_all_abstain parameter (Nightbr)

This PR was submitted for the 4.0 branch but it was squashed and merged into the 2.7 branch instead (closes #9125).

Discussion
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Add missing allow_if_all_abstain parameter

Missing documentation on access_decision_manager

Commits
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0ff32ad Add missing allow_if_all_abstain parameter

Author: javiereguiluz

security/voters.rst

@@ -266,6 +266,7 @@ security configuration:
         security:
             access_decision_manager:
                 strategy: unanimous
+                allow_if_all_abstain: true
 
     .. code-block:: xml
 
@@ -279,7 +280,7 @@ security configuration:
         >
 
             <config>
-                <access-decision-manager strategy="unanimous" />
+                <access-decision-manager strategy="unanimous" allow-if-all-abstain="true"  />
             </config>
         </srv:container>
 
@@ -289,5 +290,6 @@ security configuration:
         $container->loadFromExtension('security', array(
             'access_decision_manager' => array(
                 'strategy' => 'unanimous',
+                'allow_if_all_abstain' => true,
             ),
         ));