Follow-up from "Expose hierarchy widget definition in WorkItemType graphQL type"

We need to document in https://docs.gitlab.com/ee/development/work_items.html#write-a-database-migration that we are caching hierarchy restrictions in the DB. Cache TTL is only 10 minutes, but still might confuse developers that change hierarchy restrictions

We could document in https://docs.gitlab.com/ee/development/work_items.html#write-a-database-migration that we need to clear reactive cache on the parent type if we change hierarchy restrictions in a migration