Clarify and update documention for extra chain certificates.
PR#3878.
Reviewed-by: Rich Salz <rsalz@openssl.org>
(cherry picked from commit 2fd7fb99db)
Document that the certificate passed to SSL_CTX_add_extra_chain_cert()
should not be freed by the application.
PR#3409
(cherry picked from commit 0535c2d67c)