Improve is_granted documentation

Author: Jean85

reference/twig_reference.rst

@@ -302,15 +302,19 @@ is_granted
     {{ is_granted(role, object = null, field = null) }}
 
 ``role``
-    **type**: ``string``
+    **type**: ``string``, ``string[]``
 ``object`` *(optional)*
     **type**: ``object``
 ``field`` *(optional)*
     **type**: ``string``
 
-Returns ``true`` if the current user has the required role. Optionally,
-an object can be pasted to be used by the voter. More information can be
-found in :ref:`security-template`.
+Returns ``true`` if the current user has the required role, if only one 
+is passed; if more than one is passed, with an array, the behavior depends
+on how the :ref:`Access Decision Manager <components-security-access-decision-manager>`
+is configured; by default, the user has to have at least one of the passed roles.
+
+Optionally, an object can be pasted to be used by the voter. More information
+can be found in :ref:`security-template`.
 
 .. note::