Clarify CLI OCSP documentation

This fixes issue #3043, which ultimately was reported because
documentation was not clear on the meaning of the "-ignore_err" option.
Update both command line documentation and add this option to manpage.

Reviewed-by: Andy Polyakov <appro@openssl.org>
Reviewed-by: Rich Salz <rsalz@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/4143)