netfilter: fix nf_conntrack_helper documentation

Since kernel 4.7 this defaults to off.
Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>

Documentation/networking/nf_conntrack-sysctl.txt

@@ -62,10 +62,13 @@ nf_conntrack_generic_timeout - INTEGER (seconds)
 	protocols.
 
 nf_conntrack_helper - BOOLEAN
-	0 - disabled
-	not 0 - enabled (default)
+	0 - disabled (default)
+	not 0 - enabled
 
 	Enable automatic conntrack helper assignment.
+	If disabled it is required to set up iptables rules to assign
+	helpers to connections.  See the CT target description in the
+	iptables-extensions(8) man page for further information.
 
 nf_conntrack_icmp_timeout - INTEGER (seconds)
 	default 30