ipv6: sr: Add documentation for seg_flowlabel sysctl

This patch adds a documentation for seg_flowlabel sysctl into
Documentation/networking/ip-sysctl.txt

Signed-off-by: Ahmed Abdelsalam <amsalam20@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

Documentation/networking/ip-sysctl.txt

@@ -1428,6 +1428,19 @@ ip6frag_low_thresh - INTEGER
 ip6frag_time - INTEGER
 	Time in seconds to keep an IPv6 fragment in memory.
 
+IPv6 Segment Routing:
+
+seg6_flowlabel - INTEGER
+	Controls the behaviour of computing the flowlabel of outer
+	IPv6 header in case of SR T.encaps
+
+	-1 set flowlabel to zero.
+	0 copy flowlabel from Inner packet in case of Inner IPv6
+		(Set flowlabel to 0 in case IPv4/L2)
+	1 Compute the flowlabel using seg6_make_flowlabel()
+
+	Default is 0.
+
 conf/default/*:
 	Change the interface-specific default settings.