threat_model: Update client assumptions for unsafe calls

Signed-off-by: Tomás González <[email protected]>

src/parsec_security/rust_client_threat_model/threat_model.md

@@ -24,6 +24,8 @@ by Parsec cannot be guaranteed.
    application.
 3. The client application obtains or generates a persistent name that it uses consistently for
    accessing its service-side assets.
+4. Invoking third party libraries using unsafe calls relies on their respective threat models and
+   assurances, and is outside of the scope of the parsec client.
 
 ## Assets