Document syslog directives in sentinel.conf. (redis#11889)

Redis supports syslog integration via these directives, documented in redis.conf.
While these directives are not documented in sentinel.conf, they do work with Redis-Sentinel.
It took me a while to realize this, adding them to make it clear.

sentinel.conf

@@ -32,6 +32,16 @@ loglevel notice
 # output for logging but daemonize, logs will be sent to /dev/null
 logfile ""
 
+# To enable logging to the system logger, just set 'syslog-enabled' to yes,
+# and optionally update the other syslog parameters to suit your needs.
+# syslog-enabled no
+
+# Specify the syslog identity.
+# syslog-ident sentinel
+
+# Specify the syslog facility. Must be USER or between LOCAL0-LOCAL7.
+# syslog-facility local0
+
 # sentinel announce-ip <ip>
 # sentinel announce-port <port>
 #