netlink: specs: mptcp: fix missing doc

Two operations didn't have a small description. It looks like something
that has been missed in the original commit introducing this file.

Replace the two "todo" by a small and simple description: Create/Destroy
subflow.

While at it, also uniform the capital letters, avoid double spaces, and
fix the "announce" event description: a new "address" has been
announced, not a new "subflow".

Reviewed-by: Geliang Tang <geliang@kernel.org>
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Link: https://patch.msgid.link/20241221-net-mptcp-netlink-specs-pm-doc-fixes-v2-3-e54f2db3f844@kernel.org


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