Documentation: add description for net.sctp.ecn_enable

Describe it in networking/ip-sysctl.rst like other SCTP options.

Fixes: 2f5268a924 ("sctp: allow users to set netns ecn flag with sysctl")
Signed-off-by: Xin Long <lucien.xin@gmail.com>
Acked-by: Marcelo Ricardo Leitner <marcelo.leitner@gmail.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

Documentation/networking/ip-sysctl.rst

@@ -2950,6 +2950,18 @@ intl_enable - BOOLEAN
 
 	Default: 0
 
+ecn_enable - BOOLEAN
+        Control use of Explicit Congestion Notification (ECN) by SCTP.
+        Like in TCP, ECN is used only when both ends of the SCTP connection
+        indicate support for it. This feature is useful in avoiding losses
+        due to congestion by allowing supporting routers to signal congestion
+        before having to drop packets.
+
+        1: Enable ecn.
+        0: Disable ecn.
+
+        Default: 1
+
 
 ``/proc/sys/net/core/*``
 ========================