Fix some descriptions in cert_key_type_fix.

There were extraneous entries in the lists of signed key types: these values describe the type of the key being signed (eg, ed25519 or RSA), not the type of the certificate.

Further, clarify Tor bug tor#40124 (closed), so that people know to expect incorrect cert key types on older signing->link certs.

mentioned in issue tor#40124 (closed)

assigned to @asn

assigned to @dgoulet and unassigned @asn

merged