Merge branch '521595-document-all-migration-helpers-shared-examples' into 'master'

Document all the migration helper shared examples

See merge request https://gitlab.com/gitlab-org/gitlab/-/merge_requests/183880



Merged-by: Siddharth Dungarwal <sdungarwal@gitlab.com>
Approved-by: Siddharth Dungarwal <sdungarwal@gitlab.com>
Co-authored-by: Arturo Herrero <arturo.herrero@gmail.com>