Update SSL_CTX_sess_set_new_cb(3) docs for refcounts

Update SSL_CTX_sess_set_new_cb(3) docs for refcounts

The existing documentation for the new-session callback was unclear
about the requirements on the callback with respect to reference-handling
of the session object being created.  Be more explicit about the
(non-)requirements on the callback code for "success" (1) and "ignore"
(0) return values.

Reviewed-by: Matt Caswell <matt@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/10848)