Document some `import_` fields in the Projects API

What does this MR do and why?

The added import_ fields are exposed with the API::Entities::Project REST API entity. See https://gitlab.com/gitlab-org/gitlab/-/blob/master/lib/api/entities/project.rb#L99

This will confidently allow users to actually rely on this field.

Refs:

• terraform-provider-gitlab!1431 (closed)
• https://github.com/xanzy/go-gitlab/pull/1636

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.