Document X509_check_private_key and relative

Document two private key check functions:

X509_check_private_key
X509_REQ_check_private_key

Signed-off-by: Paul Yang <paulyang.inf@gmail.com>

Reviewed-by: Rich Salz <rsalz@openssl.org>
Reviewed-by: Matt Caswell <matt@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/3614)