Added tip regarding VoterInterface restrictions

Added tip about supportsAttribute and supportsClass
fixes #2493

Author: peterrehm

cookbook/security/voters.rst

@@ -100,6 +100,14 @@ and compare the IP address against a set of blacklisted IP addresses:
 That's it! The voter is done. The next step is to inject the voter into
 the security layer. This can be done easily through the service container.
 
+.. tip::
+
+   The methods supportsAttribute() and supportsClass() are not being called
+   internally by the framework. Once you have registered your voter the vote()
+   method will always be called, not depending on your restrictions. Therefore
+   you need to call those methods in your implementation of the vote() method.
+   
+
 Declaring the Voter as a Service
 --------------------------------