Update draft-toutain-schc-access-control-00.md

[MarinoMtz]

• Adding some attack senarios
• Changing end-point instead of entities so that is the same terminology as in the archi draft. Deleating "core instance" -- I think saying core is enough. The archi draft says: "A session called a SCHC Instance is established and SCHC maintains a state and timers associated to that Instance."