Documentation: add description for net.sctp.reconf_enable

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

Fixes: c0d8bab6 ("sctp: add get and set sockopt for reconf_enable")
Signed-off-by: NXin Long <lucien.xin@gmail.com>
Acked-by: NMarcelo Ricardo Leitner <marcelo.leitner@gmail.com>
Signed-off-by: NJakub Kicinski <kuba@kernel.org>

Documentation/networking/ip-sysctl.rst

@@ -2925,6 +2925,17 @@ plpmtud_probe_interval - INTEGER
 
 	Default: 0
 
+reconf_enable - BOOLEAN
+        Enable or disable extension of Stream Reconfiguration functionality
+        specified in RFC6525. This extension provides the ability to "reset"
+        a stream, and it includes the Parameters of "Outgoing/Incoming SSN
+        Reset", "SSN/TSN Reset" and "Add Outgoing/Incoming Streams".
+
+	- 1: Enable extension.
+	- 0: Disable extension.
+
+	Default: 0
+
 
 ``/proc/sys/net/core/*``
 ========================