minor #7279 Update voters.rst (ErikRoelofs, javiereguiluz)

This PR was submitted for the 3.2 branch but it was merged into the 2.8 branch instead (closes #7279).

Discussion
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Update voters.rst

Made clear the difference between using denyAccessUnlessGranted and isGranted - the former is not just a shortcut to calling isGranted.

Commits
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4023044 Minor reword
188b654 Update voters.rst

Author: xabbuh

security/voters.rst

@@ -102,8 +102,10 @@ decides this using whatever logic you want.
 
     The ``denyAccessUnlessGranted()`` and ``isGranted()`` functions are both
     just shortcuts of the ``Controller`` class to call ``isGranted()`` on
-    the ``security.authorization_checker`` service.
-
+    the ``security.authorization_checker`` service. The main difference is that
+    when access is not granted, ``denyAccessUnlessGranted()`` throws an
+    ``AccessDeniedException``, whereas ``isGranted()`` returns ``false``.
+    
 Creating the custom Voter
 -------------------------