ipv6: add documentation for stable_secret, idgen_delay and idgen_retries knobs

Cc: Erik Kline <[e-mail]>
Cc: Fernando Gont <[e-mail]>
Cc: Lorenzo Colitti <[e-mail]>
Cc: YOSHIFUJI Hideaki/吉藤英明 <[e-mail]>
Signed-off-by: Hannes Frederic Sowa <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

Documentation/networking/ip-sysctl.txt

···
 	FALSE: disabled
 	Default: FALSE
 
+idgen_delay - INTEGER
+	Controls the delay in seconds after which time to retry
+	privacy stable address generation if a DAD conflict is
+	detected.
+	Default: 1 (as specified in RFC7217)
+
+idgen_retries - INTEGER
+	Controls the number of retries to generate a stable privacy
+	address if a DAD conflict is detected.
+	Default: 3 (as specified in RFC7217)
+
 mld_qrv - INTEGER
 	Controls the MLD query robustness variable (see RFC3810 9.1).
 	Default: 2 (as specified by RFC3810 9.1)
···
 	address selection algorithm.
 		0: disabled (default)
 		1: enabled
+
+stable_secret - IPv6 address
+	This IPv6 address will be used as a secret to generate IPv6
+	addresses for link-local addresses and autoconfigured
+	ones. All addresses generated after setting this secret will
+	be stable privacy ones by default. This can be changed via the
+	addrgenmode ip-link. conf/default/stable_secret is used as the
+	secret for the namespace, the interface specific ones can
+	overwrite that. Writes to conf/all/stable_secret are refused.
+
+	It is recommended to generate this secret during installation
+	of a system and keep it stable after that.
+
+	By default the stable secret is unset.
 
 icmp/*:
 ratelimit - INTEGER