man: mptcp: 'backup' flag also affects outgoing data

man: mptcp: 'backup' flag also affects outgoing data

That's the behaviour with the default packet scheduler.

In some early design, the default scheduler was supposed to take into
account only the received backup flags, but it ended up not being the
case, and setting the flag would also affect outgoing data.

Suggested-by: Mat Martineau <martineau@kernel.org>
Acked-by: Mat Martineau <martineau@kernel.org>
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Signed-off-by: Stephen Hemminger <stephen@networkplumber.org>