Fix robe of the dead mage description

It was false.
Better unnaturally aware message

The game should never say something like "The demonic assassin does not
notice you." Now it will say "The demonic assassin slowly turns in your
direction." -- much more appropriate for a creature guaranteed to notice
you on the next turn.

(You can be undetected with the demon in the room if it gets summoned
while you were there. It will only last until your next turn, of
course.)