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>

docs/threat_model/threat_model.rst

@@ -36,6 +36,9 @@ assumptions:
 - There is no Secure-EL2. We don't consider threats that may come with
   Secure-EL2 software.
 
+- No experimental features are enabled. We do not consider threats that may come
+  from them.
+
 Data Flow Diagram
 =================