cert-spec.txt contains incomplete reference / documentation for certificate types

The cert-spec.txt document references various Certificate types in a section 4.2, but there exists no such complete documentation in any of the -spec.txt files.

The section 4.2 that it appears to reference is in prop 220 which includes specs for each of the cert types mentioned:

• Link key certificate certified by Ed25519 signing key
• Ed25519 TLS authentication key certified by Ed25519 signing key
• RSA cross-certificate for Ed25519 identity key