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>