Merge branch 'patch-28' into 'master'

Update index.md

See merge request gitlab-org/gitlab-ce!18450

doc/user/project/clusters/index.md

@@ -238,6 +238,7 @@ work.
 The default environment scope is `*`, which means all jobs, regardless of their
 environment, will use that cluster. Each scope can only be used by a single
 cluster in a project, and a validation error will occur if otherwise.
+Also, jobs that don't have an environment keyword set will not be able to access any cluster.
 
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