doc: mptcp: alphabetical order

doc: mptcp: alphabetical order

Similar to what is done in other 'sysctl' pages: it looks clearer from a
readability perspective.

This might cause troubles in the short/mid-term with the backports, but
by not putting new entries at the end, this can help to reduce conflicts
in case of backports in the long term. We don't change the information
here too often, so it looks OK to do that.

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-v3-2-e94cdd9f2673@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>