ipvs: Document sysctl sync_qlen_max and sync_sock_size

Document sysctl sync_qlen_max and sync_sock_size based on
commit 1c003b1580 ("ipvs: wakeup master thread").

Signed-off-by: Hangbin Liu <liuhangbin@gmail.com>
Signed-off-by: Simon Horman <horms@verge.net.au>

Documentation/networking/ipvs-sysctl.txt

@@ -217,6 +217,20 @@ sync_retries - INTEGER
 	to protect against loss of sync messages. The range of the
 	sync_retries is from 0 to 3.
 
+sync_qlen_max - UNSIGNED LONG
+
+	Hard limit for queued sync messages that are not sent yet. It
+	defaults to 1/32 of the memory pages but actually represents
+	number of messages. It will protect us from allocating large
+	parts of memory when the sending rate is lower than the queuing
+	rate.
+
+sync_sock_size - INTEGER
+	default 0
+
+	Configuration of SNDBUF (master) or RCVBUF (slave) socket limit.
+	Default value is 0 (preserve system defaults).
+
 snat_reroute - BOOLEAN
 	0 - disabled
 	not 0 - enabled (default)