Sentinel: clarify announce-ip/port options in sentinel.conf.

sentinel.conf

@@ -4,12 +4,27 @@
 # The port that this sentinel instance will run on
 port 26379
 
-# announce <ip>
-# The IP that other sentinels use to connect to this sentinel instance
+# sentinel announce-ip <ip>
+# sentinel announce-port <port>
+#
+# The above two configuration directives are useful in environments where,
+# because of NAT, Sentinel is reachable from outside via a non-local address.
+#
+# When announce-ip is provided, the Sentinel will claim the specified IP address
+# in HELLO messages used to gossip its presence, instead of auto-detecting the
+# local address as it usually does.
+#
+# Similarly when announce-port is provided and is valid and non-zero, Sentinel
+# will announce the specified TCP port.
+#
+# The two options don't need to be used together, if only announce-ip is
+# provided, the Sentinel will announce the specified IP and the server port
+# as specified by the "port" option. If only announce-port is provided, the
+# Sentinel will announce the auto-detected local IP and the specified port.
 #
 # Example:
 #
-# sentinel announce 1.2.3.4
+# sentinel announce-ip 1.2.3.4
 
 # dir <working-directory>
 # Every long running process should have a well-defined working directory.