Add description in X509_STORE manipulation

Add memory management description in X509_STORE_add_cert, otherwise
users will not be aware that they are leaking memory...

Reviewed-by: Dmitry Belyavskiy <beldmit@gmail.com>
Reviewed-by: Matthias St. Pierre <Matthias.St.Pierre@ncp-e.com>
Reviewed-by: Matt Caswell <matt@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/9484)

(cherry picked from commit 86333b6e0c5c488130ab237e95b8520891b81bf6)