Fix wrong docs on untagged jobs

What does this MR do?

Cleans up some wrong docs we had about how Runners picked untagged jobs.

Seems the tagged parameter from the Runner was removed in gitlab-runner!896 (diffs).

Related issues

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/47387

Author's checklist

• Apply the correct labels and milestone

Review checklist

• PM's review (recommended, but not a blocker)
• Technical writer's review (required)