Clarify the prerequisite of import to GitLab self-managed

What does this MR do and why?

The "When migrating to GitLab self-managed" section of the Migrate projects and groups by using file exports documentation states:

• An administrator must perform the import with an administrator access token.

However, "with an administrator access token" is a bit misleading because users do not use the access token when they import projects from Web UI.

So, people who read this statement may think "Should we import a project by using Project import and export API with access token, and we shouldn't do it by Web UI".

(I have confirmed in my test environment that the contributions and users are mapped when the Administrator imported the project from the Web UI)

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