docs(threat-model): club RME note with other assumptions

The fact that RME is out of the generic threat model's scope is just
another assumption we make about the target of evaluation so mention
it there.

Signed-off-by: Sandrine Bailleux <sandrine.bailleux@arm.com>
Change-Id: I839ec5427f36b085148338030e8b1b85191d4245
1 file changed