Sentinel: document auth-user directive.

sentinel.conf

@@ -102,6 +102,18 @@ sentinel monitor mymaster 127.0.0.1 6379 2
 #
 # sentinel auth-pass mymaster MySUPER--secret-0123passw0rd
 
+# sentinel auth-user <master-name> <username>
+#
+# This is useful in order to authenticate to instances having ACL capabilities,
+# that is, running Redis 6.0 or greater. When just auth-pass is provided the
+# Sentinel instance will authenticate to Redis using the old "AUTH <pass>"
+# method. When also an username is provided, it will use "AUTH <user> <pass>".
+# In the Redis servers side, the ACL to provide just minimal access to
+# Sentinel instances, should be configured along the following lines:
+#
+#     user sentinel-user >somepassword +client +subscribe +publish \
+#                        +ping +info +multi +slaveof +config +client +exec on
+
 # sentinel down-after-milliseconds <master-name> <milliseconds>
 #
 # Number of milliseconds the master (or any attached replica or sentinel) should