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)

doc/man3/X509_STORE_add_cert.pod

@@ -55,7 +55,9 @@ operate on pointers to B<X509> objects, though.
 
 X509_STORE_add_cert() and X509_STORE_add_crl() add the respective object
 to the B<X509_STORE>'s local storage.  Untrusted objects should not be
-added in this way.
+added in this way.  The added object's reference count is incremented by one,
+hence the caller retains ownership of the object and needs to free it when it
+is no longer needed.
 
 X509_STORE_set_depth(), X509_STORE_set_flags(), X509_STORE_set_purpose(),
 X509_STORE_set_trust(), and X509_STORE_set1_param() set the default values
@@ -90,7 +92,7 @@ L<X509_STORE_get0_param(3)>
 
 =head1 COPYRIGHT
 
-Copyright 2017-2018 The OpenSSL Project Authors. All Rights Reserved.
+Copyright 2017-2019 The OpenSSL Project Authors. All Rights Reserved.
 
 Licensed under the Apache License 2.0 (the "License").  You may not use
 this file except in compliance with the License.  You can obtain a copy