docs: Document the implemented composite signature+hash algorithms

docs: Document the implemented composite signature+hash algorithms

The details for RSA and EdDSA have already been documented, albeit the
RSA documentation wasn't conforming properly to the POD format.

Reviewed-by: Matt Caswell <matt@openssl.org>
Reviewed-by: Tomas Mraz <tomas@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/25422)