Clarify docs.

Document that the certificate passed to SSL_CTX_add_extra_chain_cert()
should not be freed by the application.

PR#3409

doc/ssl/SSL_CTX_add_extra_chain_cert.pod

@@ -24,6 +24,8 @@ the library will try to complete the chain from the available CA
 certificates in the trusted CA storage, see
 L<SSL_CTX_load_verify_locations(3)|SSL_CTX_load_verify_locations(3)>.
 
+The B<x509> certificate provided to SSL_CTX_add_extra_chain_cert() will be freed by the library when the B<SSL_CTX> is destroyed. An application B<should not> free the B<x509> object.
+
 =head1 RESTRICTIONS
 
 Only one set of extra chain certificates can be specified per SSL_CTX