Fix EC_KEY_set_private_key() NULL priv_key docs

Updates the docs to describe EC_KEY_set_private_key() function behavior
when a NULL priv_key argument is passed.

Reviewed-by: Nicola Tuveri <nic.tuv@gmail.com>
Reviewed-by: Todd Short <todd.short@me.com>
(Merged from https://github.com/openssl/openssl/pull/18941)