docs(threat-model): make experimental features out of scope

By nature, experimental features are incomplete pieces of work,
sometimes going under rapid change. Typically, the threat model
implications have not been fully considered yet.

Change-Id: Ice8d4273a789558e912f82cde592da4747b37fdf
Signed-off-by: Sandrine Bailleux <sandrine.bailleux@arm.com>