Clearify comment for server_name field in Mixer oop auth config

policy/v1beta1/cfg.proto

@@ -490,8 +490,8 @@ message Tls {
     string custom_header = 5;
   }
 
-  // Indicates the name of adapter backend which is useful for routing with
-  // proxy-fronted backend.
+  // Used to configure mixer TLS client to verify the hostname on the returned
+  // certificates. It is also included in the client's handshake to support SNI.
   string server_name = 6;
 }
 
@@ -530,7 +530,8 @@ message Mutual {
   // include Istio CA certificates and system certificates in cert pool.
   string ca_certificates = 3;
 
-  // Indicates the name of adapter backend server which is useful for routing with
-  // proxy-fronted backend.
+  // Used to configure mixer mutual TLS client to supply server name for SNI.
+  // It is not used to verify the hostname of the peer certificate, since 
+  // Istio verifies whitelisted SAN fields in mutual TLS.
   string server_name = 4;
 }