Resolve arch-security-consideration-use-psk

Resolves #900. Resolves #908

Remove at-risk assertion arch-security-consideration-use-psk
Keep statement, but downgrade (SHOULD -> should) to an informative statement
Address Issue #900 by using "certificates" instead of "pre-shared keys"
Edit some of the following text to more clearly differentiate between the roles of certificates (which prove identity) and other security materials (like API keys or passwords) that are used for access control.
Since it was an adjacent issue, also resolve #908 about guest networks

w3c / wot-architecture