Documentation: add documentation of ping_group_range

Support for non-root users to send ICMP ECHO requests was added
back in Linux 3.0 kernel, but the documentation for the sysctl
to enable it has been missing.
Signed-off-by: Stephen Hemminger <stephen@networkplumber.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

Documentation/networking/ip-sysctl.txt

@@ -983,6 +983,13 @@ ip_early_demux - BOOLEAN
 	reduces overall throughput, in such case you should disable it.
 	Default: 1
 
+ping_group_range - 2 INTEGERS
+	Restrict ICMP_PROTO datagram sockets to users in the group range.
+	The default is "1 0", meaning, that nobody (not even root) may
+	create ping sockets.  Setting it to "100 100" would grant permissions
+	to the single group. "0 4294967295" would enable it for the world, "100
+	4294967295" would enable it for the users, but not daemons.
+
 tcp_early_demux - BOOLEAN
 	Enable early demux for established TCP sockets.
 	Default: 1