docs: networking: mptcp: Add MPTCP sysctl entries

Describe the two MPTCP sysctls, what the values mean, and the default
settings.
Acked-by: Geliang Tang <geliangtang@gmail.com>
Signed-off-by: Mat Martineau <mathew.j.martineau@linux.intel.com>
Reviewed-by: Matthieu Baerts <matthieu.baerts@tessares.net>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

Documentation/networking/index.rst

@@ -70,6 +70,7 @@ Contents:
    lapb-module
    mac80211-injection
    mpls-sysctl
+   mptcp-sysctl
    multiqueue
    netconsole
    netdev-features

Documentation/networking/mptcp-sysctl.rst

+.. SPDX-License-Identifier: GPL-2.0
+
+=====================
+MPTCP Sysfs variables
+=====================
+
+/proc/sys/net/mptcp/* Variables
+===============================
+
+enabled - INTEGER
+	Control whether MPTCP sockets can be created.
+
+	MPTCP sockets can be created if the value is nonzero. This is
+	a per-namespace sysctl.
+
+	Default: 1
+
+add_addr_timeout - INTEGER (seconds)
+	Set the timeout after which an ADD_ADDR control message will be
+	resent to an MPTCP peer that has not acknowledged a previous
+	ADD_ADDR message.
+
+	The default value matches TCP_RTO_MAX. This is a per-namespace
+	sysctl.
+
+	Default: 120

MAINTAINERS

@@ -12265,6 +12265,7 @@ L:	mptcp@lists.01.org
 S:	Maintained
 W:	https://github.com/multipath-tcp/mptcp_net-next/wiki
 B:	https://github.com/multipath-tcp/mptcp_net-next/issues
+F:	Documentation/networking/mptcp-sysctl.rst
 F:	include/net/mptcp.h
 F:	include/uapi/linux/mptcp.h
 F:	net/mptcp/