These two value strings are only for displaying the contents of the
two octets contained in a signature_algorithms extension as a TLS 1.2
SignatureAndHashAlgorithm. (As opposed to displaying it as a TLS 1.3
SignatureScheme where the two octets do not necessarily separately
indicate signature and hash.)

SM3 and SM2 are not assigned in those registries, are not supported in
TLS 1.2 (RFC 8998), and if they were would not receive the values chosen
here.

Add a comment explaining why even though RFC 8422 registered ED25519
and ED448 to the TLS 1.2 SignatureAlgorithm and HashAlgorithm registries
we don't add it to the value strings for those. (It would take more
complicated processing to avoid confusion, and for little gain.)

Fix #19801