Sentinel: added documentation about slave-priority in redis.conf

redis.conf

@@ -196,6 +196,21 @@ slave-read-only yes
 #
 # repl-timeout 60
 
+# The slave priority is an integer number published by Redis in the INFO output.
+# It is used by Redis Sentinel in order to select a slave to promote into a
+# master if the master is no longer working correctly.
+#
+# A slave with a low priority number is considered better for promotion, so
+# for instance if there are three slaves with priority 10, 100, 25 Sentinel will
+# pick the one wtih priority 10, that is the lowest.
+#
+# However a special priority of 0 marks the slave as not able to perform the
+# role of master, so a slave with priority of 0 will never be selected by
+# Redis Sentinel for promotion.
+#
+# By default the priority is 100.
+slave-priority 100
+
 ################################## SECURITY ###################################
 
 # Require clients to issue AUTH <PASSWORD> before processing any other