RT4354: Add some cross-refs

Stack Overflow has a number of questions related to mutual authentication,
the client and its certificate. Those visiting the man pages for functions
like SSL_CTX_use_certificate and SSL_CTX_load_verify_locations don't
receive the benefit of a cross reference to SSL_CTX_set_client_CA_list.

Reviewed-by: Richard Levitte <levitte@openssl.org>