doc: mptcp: net.mptcp.pm_type is deprecated

The net.mptcp.pm_type sysctl knob has been deprecated in v6.15,
net.mptcp.path_manager should be used instead.

Adapt the section about path managers to suggest using the new sysctl
knob instead of the deprecated one.

Fixes: 595c26d ("mptcp: sysctl: set path manager by name")
Cc: stable@vger.kernel.org
Reviewed-by: Geliang Tang <geliang@kernel.org>
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Reviewed-by: Simon Horman <horms@kernel.org>
Link: https://patch.msgid.link/20250908-net-mptcp-misc-fixes-6-17-rc5-v1-2-5f2168a66079@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

Author: matttbe

Documentation/networking/mptcp.rst

@@ -60,10 +60,10 @@ address announcements. Typically, it is the client side that initiates subflows,
 and the server side that announces additional addresses via the ``ADD_ADDR`` and
 ``REMOVE_ADDR`` options.
 
-Path managers are controlled by the ``net.mptcp.pm_type`` sysctl knob -- see
-mptcp-sysctl.rst. There are two types: the in-kernel one (type ``0``) where the
-same rules are applied for all the connections (see: ``ip mptcp``) ; and the
-userspace one (type ``1``), controlled by a userspace daemon (i.e. `mptcpd
+Path managers are controlled by the ``net.mptcp.path_manager`` sysctl knob --
+see mptcp-sysctl.rst. There are two types: the in-kernel one (``kernel``) where
+the same rules are applied for all the connections (see: ``ip mptcp``) ; and the
+userspace one (``userspace``), controlled by a userspace daemon (i.e. `mptcpd
 <https://mptcpd.mptcp.dev/>`_) where different rules can be applied for each
 connection. The path managers can be controlled via a Netlink API; see
 netlink_spec/mptcp_pm.rst.