minor #8247 Improved a help note about authentication providers (javiereguiluz)

This PR was squashed before being merged into the 2.7 branch (closes #8247).

Discussion
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Improved a help note about authentication providers

This should fix #5264.

Commits
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bf26484 Improved a help note about authentication providers

Author: xabbuh

security/api_key_authentication.rst

@@ -111,8 +111,11 @@ method, if any.
 
 .. caution::
 
-    In case you return ``null`` from your ``createToken()`` method, be sure to enable
-    ``anonymous`` in you firewall. This way you'll be able to get an ``AnonymousToken``.
+    In case you return ``null`` from your ``createToken()`` method, Symfony
+    passes this request to the next authentication provider. If you haven't
+    configured any other provider, enable the ``anonymous`` option in your
+    firewall. This way Symfony executes the anonymous authentication provider
+    and you'll get an ``AnonymousToken``.
 
 2. supportsToken
 ~~~~~~~~~~~~~~~~