Merge branch 'mc/doc/variable-precedence-docs' into 'master'

Document multi-project variable precedence

Closes gitlab-ee#12879

See merge request gitlab-org/gitlab-ce!31291

doc/ci/multi_project_pipelines.md

@@ -171,6 +171,11 @@ In this scenario, the `UPSTREAM_BRANCH` variable with a value related to the
 upstream pipeline will be passed to the `downstream-job` job, and will be available
 within the context of all downstream builds.
+NOTE: **Tip:**
+Upstream pipelines take precedence over downstream ones. If there are two
+variables with the same name defined in both upstream and downstream projects,
+the ones defined in the upstream project will take precedence.
+
 ### Limitations
 Because bridge jobs are a little different to regular jobs, it is not