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

(cherry picked from commit 86333b6e0c5c488130ab237e95b8520891b81bf6)

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 OpenSSL license (the "License").  You may not use
 this file except in compliance with the License.  You can obtain a copy