Merge branch 'mc/update-pre-post-stages-documentation' into 'master'

Update documentation of `.pre` and `.post` stages with regards to PEP

See merge request https://gitlab.com/gitlab-org/gitlab/-/merge_requests/181382



Merged-by: Marcel Amirault <mamirault@gitlab.com>
Approved-by: Marcel Amirault <mamirault@gitlab.com>
Reviewed-by: Marcel Amirault <mamirault@gitlab.com>
Co-authored-by: Martin Čavoj <mcavoj@gitlab.com>