ipv6: Update ip-sysctl.txt documentation for recent changes to accept_ra and forwarding

Documentation for recent changes to the tunables accept_ra and
forwarding.
Signed-off-by: Thomas Graf <tgraf@infradead.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

Documentation/networking/ip-sysctl.txt

@@ -1014,6 +1014,12 @@ conf/interface/*:
 accept_ra - BOOLEAN
 	Accept Router Advertisements; autoconfigure using them.
 
+	Possible values are:
+		0 Do not accept Router Advertisements.
+		1 Accept Router Advertisements if forwarding is disabled.
+		2 Overrule forwarding behaviour. Accept Router Advertisements
+		  even if forwarding is enabled.
+
 	Functional default: enabled if local forwarding is disabled.
 			    disabled if local forwarding is enabled.
 
@@ -1075,7 +1081,12 @@ forwarding - BOOLEAN
 	Note: It is recommended to have the same setting on all
 	interfaces; mixed router/host scenarios are rather uncommon.
 
-	FALSE:
+	Possible values are:
+		0 Forwarding disabled
+		1 Forwarding enabled
+		2 Forwarding enabled (Hybrid Mode)
+
+	FALSE (0):
 
 	By default, Host behaviour is assumed.  This means:
 
@@ -1085,18 +1096,24 @@ forwarding - BOOLEAN
 	   Advertisements (and do autoconfiguration).
 	4. If accept_redirects is TRUE (default), accept Redirects.
 
-	TRUE:
+	TRUE (1):
 
 	If local forwarding is enabled, Router behaviour is assumed.
 	This means exactly the reverse from the above:
 
 	1. IsRouter flag is set in Neighbour Advertisements.
 	2. Router Solicitations are not sent.
-	3. Router Advertisements are ignored.
+	3. Router Advertisements are ignored unless accept_ra is 2.
 	4. Redirects are ignored.
 
-	Default: FALSE if global forwarding is disabled (default),
-		 otherwise TRUE.
+	TRUE (2):
+
+	Hybrid mode. Same behaviour as TRUE, except for:
+
+	2. Router Solicitations are being sent when necessary.
+
+	Default: 0 (disabled) if global forwarding is disabled (default),
+		 otherwise 1 (enabled).
 
 hop_limit - INTEGER
 	Default Hop Limit to set.