Move the x509_V_ERR_xxx definitions from openssl-verify to
X509_STORE_CTX_get_error.pod.  Add some missing ones.  Consistently
start with a lowercase letter, unless it's an acronym.

Fix some markup mistakes in X509_verify_cert.
Reviewed-by: NDmitry Belyavskiy <beldmit@gmail.com>
Reviewed-by: NMatthias St. Pierre <Matthias.St.Pierre@ncp-e.com>
(Merged from https://github.com/openssl/openssl/pull/10132)