Merge branch '520478-update-documentation-for-creating-pipeline-execution-policies' into 'master'

Improve pipeline execution policy documentation

See merge request https://gitlab.com/gitlab-org/gitlab/-/merge_requests/182269



Merged-by: Ryan Lehmann <rlehmann@gitlab.com>
Reviewed-by: Martin Čavoj <mcavoj@gitlab.com>
Reviewed-by: Ryan Lehmann <rlehmann@gitlab.com>
Co-authored-by: Martin Čavoj <mcavoj@gitlab.com>