Reword documentation for {SCT_CTX/CT_POLICY_EVAL_CTX}_set_time

Do not call the time "current", as a different time can be provided.
For example, a time slightly in the future, to provide tolerance for
CT logs with a clock that is running fast.

Reviewed-by: Viktor Dukhovni <viktor@openssl.org>
Reviewed-by: Rich Salz <rsalz@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/1554)