Documentation: add description for net.sctp.intl_enable

Describe it in networking/ip-sysctl.rst like other SCTP options.
We need to document this especially as when using the feature
of User Message Interleaving, some socket options also needs
to be set.

Fixes: 463118c ("sctp: support sysctl to allow users to use stream interleave")
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

@@ -2936,6 +2936,20 @@ reconf_enable - BOOLEAN
 
 	Default: 0
 
+intl_enable - BOOLEAN
+        Enable or disable extension of User Message Interleaving functionality
+        specified in RFC8260. This extension allows the interleaving of user
+        messages sent on different streams. With this feature enabled, I-DATA
+        chunk will replace DATA chunk to carry user messages if also supported
+        by the peer. Note that to use this feature, one needs to set this option
+        to 1 and also needs to set socket options SCTP_FRAGMENT_INTERLEAVE to 2
+        and SCTP_INTERLEAVING_SUPPORTED to 1.
+
+	- 1: Enable extension.
+	- 0: Disable extension.
+
+	Default: 0
+
 
 ``/proc/sys/net/core/*``
 ========================