Documentation: ip-sysctl.txt: document addr_gen_mode

addr_gen_mode was introduced in without documentation, add it now.

Fixes: d35a00b8e33d ("net/ipv6: allow sysctl to change link-local address generation mode")
Signed-off-by: Sabrina Dubroca <sd@queasysnail.net>
Reviewed-by: David Ahern <dsahern@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

Documentation/networking/ip-sysctl.txt

@@ -1834,6 +1834,16 @@ stable_secret - IPv6 address
 
 	By default the stable secret is unset.
 
+addr_gen_mode - INTEGER
+	Defines how link-local and autoconf addresses are generated.
+
+	0: generate address based on EUI64 (default)
+	1: do no generate a link-local address, use EUI64 for addresses generated
+	   from autoconf
+	2: generate stable privacy addresses, using the secret from
+	   stable_secret (RFC7217)
+	3: generate stable privacy addresses, using a random secret if unset
+
 drop_unicast_in_l2_multicast - BOOLEAN
 	Drop any unicast IPv6 packets that are received in link-layer
 	multicast (or broadcast) frames.