Improve Maximum Packet Size Considerations text

[markusa]

Olivier's comment in https://mailarchive.ietf.org/arch/msg/tsvwg/T-9P8iVoU5i89VK9QJXnOG_IsL4/

If MPDCCP uses the minimum MPS accross all paths, there is a risk that 
an on-path attack on one path forces the utilization of very small 
packets on all paths. MPDCCP should be able to detect this attack and 
drop paths with a too low MPS

[gorryfair]

I suggest the security considerations could say something like: If MPDCCP uses a minimum MPS across all paths, any change to one path impacts the sender for all paths. To mitigate attacks that seek to force a low MPS, MPDCCP could detect an attempt to reduce the MPS less than a minimum MPS, and then stop using these paths.