igmp: Document sysctl force_igmp_version

There is some difference between force_igmp_version and force_mld_version.
Add document to make users aware of this.
Signed-off-by: Hangbin Liu <liuhangbin@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

Documentation/networking/ip-sysctl.txt

@@ -967,6 +967,21 @@ igmp_qrv - INTEGER
 	Default: 2 (as specified by RFC2236 8.1)
 	Minimum: 1 (as specified by RFC6636 4.5)
 
+force_igmp_version - INTEGER
+	0 - (default) No enforcement of a IGMP version, IGMPv1/v2 fallback
+	    allowed. Will back to IGMPv3 mode again if all IGMPv1/v2 Querier
+	    Present timer expires.
+	1 - Enforce to use IGMP version 1. Will also reply IGMPv1 report if
+	    receive IGMPv2/v3 query.
+	2 - Enforce to use IGMP version 2. Will fallback to IGMPv1 if receive
+	    IGMPv1 query message. Will reply report if receive IGMPv3 query.
+	3 - Enforce to use IGMP version 3. The same react with default 0.
+
+	Note: this is not the same with force_mld_version because IGMPv3 RFC3376
+	Security Considerations does not have clear description that we could
+	ignore other version messages completely as MLDv2 RFC3810. So make
+	this value as default 0 is recommended.
+
 conf/interface/*  changes special settings per interface (where
 "interface" is the name of your network interface)