Merge branch '2.8' into 3.4

* 2.8:
  Mentioned user_checkers in the main security config reference
  If multiple guard authenticators have different providers, link to the details on chaining providers together.

Author: javiereguiluz

reference/configuration/security.rst

@@ -479,6 +479,15 @@ multiple firewalls, the "context" could actually be shared:
     ignored and you won't be able to authenticate on multiple firewalls at the
     same time.
 
+User Checkers
+-------------
+
+During the authentication of a user, additional checks might be required to
+verify if the identified user is allowed to log in. Each firewall can include
+a ``user_checker`` option to define the service used to perform those checks.
+
+Learn more about user checkers in :doc:`/security/user_checkers`.
+
 HTTP-Digest Authentication
 --------------------------
 

security/multiple_guard_authenticators.rst

@@ -78,6 +78,9 @@ This is how your security configuration can look in action:
             ),
         ));
 
+If your authenticators need separate providers, you will need to create a 
+:doc:`chain of user providers </security/multiple_user_providers>`.
+
 There is one limitation with this approach - you have to use exactly one entry point.
 
 Multiple Authenticators with Separate Entry Points