net: ipv6: mld: document force_mld_version in ip-sysctl.txt

Document force_mld_version parameter in ip-sysctl.txt.
Signed-off-by: NDaniel Borkmann <dborkman@redhat.com>
Cc: Hannes Frederic Sowa <hannes@stressinduktion.org>
Acked-by: NHannes Frederic Sowa <hannes@stressinduktion.org>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

Documentation/networking/ip-sysctl.txt

@@ -1358,6 +1358,11 @@ mldv2_unsolicited_report_interval - INTEGER
 	MLDv2 report retransmit will take place.
 	Default: 1000 (1 second)
 
+force_mld_version - INTEGER
+	0 - (default) No enforcement of a MLD version, MLDv1 fallback allowed
+	1 - Enforce to use MLD version 1
+	2 - Enforce to use MLD version 2
+
 suppress_frag_ndisc - INTEGER
 	Control RFC 6980 (Security Implications of IPv6 Fragmentation
 	with IPv6 Neighbor Discovery) behavior: