improve docs on gitlab-ci-token user

What does this MR do?

Improve docs on gitlab-ci-token user

Are there points in the code the reviewer needs to double check?

No

Why was this MR needed?

Confusion around what gitlab-ci-token is, e.g. see https://stackoverflow.com/questions/37468084/what-is-the-special-gitlab-ci-token-user

Does this MR meet the acceptance criteria?

• CHANGELOG entry added
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the style guides
• Branch has no merge conflicts with master (if you do - rebase it please)
• Squashed related commits together