Add documentation for X509_cmp and related APIs

Fixes: #9088

Functions documented in this commit: X509_cmp, X509_NAME_cmp,
X509_issuer_and_serial_cmp, X509_issuer_name_cmp, X509_subject_name_cmp,
X509_CRL_cmp, X509_CRL_match

Reviewed-by: Matt Caswell <matt@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/9091)

(cherry picked from commit 4bfe304ea85ed4b2b00dd0857ccf9bdeba4ce7b5)