doc: mptcp: add missing 'available_schedulers' entry

doc: mptcp: add missing 'available_schedulers' entry

This sysctl knob has been added recently, but the documentation has not
been updated.

This knob is used to show the available schedulers choices that are
registered, similar to 'net.ipv4.tcp_available_congestion_control'.

Fixes: 73c900aa3660 ("mptcp: add net.mptcp.available_schedulers")
Reviewed-by: Mat Martineau <martineau@kernel.org>
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Link: https://lore.kernel.org/r/20240530-upstream-net-20240520-mptcp-doc-v3-1-e94cdd9f2673@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>