Merge branch 'docs-update-pipeline-variable-note' into 'master'

Fix details for pipeline variable breaking change

See merge request https://gitlab.com/gitlab-org/gitlab/-/merge_requests/173363



Merged-by: Fabio Pitino <fpitino@gitlab.com>
Approved-by: Fabio Pitino <fpitino@gitlab.com>
Co-authored-by: Marcel Amirault <mamirault@gitlab.com>