Fix `limitations` in `gitlab-runner` repo

What does this MR do?

Fixes some consistency issues in the gitlab-runner repo by:

• Updating limitations to known issues
• Incorporating that information elsewhere when it makes more sense

Related guidance

• https://docs.gitlab.com/ee/development/documentation/styleguide/word_list.html#limitations

Related issues

• gitlab#504959
• gitlab#330052 (closed)

Why was this MR needed?

What's the best way to test this MR?

What are the relevant issue numbers?