Add information that artifacts can only exist relative to the build directory

As explained in https://gitlab.com/gitlab-org/gitlab-ce/issues/15530.
A note that the declaration of an absolute directory triggers a
misleading error message and that following job(s) can't find the file
is important to avoid large scale time waisting until the issue is
fixed.

Originally submitted at
https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/6838

.

Signed-off-by: Rémy Coutable <remy@rymai.me>