Fix documentation of X509_VERIFY_PARAM_add0_policy()

The function was incorrectly documented as enabling policy checking.

Fixes: CVE-2023-0466

Reviewed-by: Paul Dale <pauli@openssl.org>
Reviewed-by: Matt Caswell <matt@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/20561)
