clarify runners currently online text

What does this MR do?

Clarify runners currently online text as decided on at https://gitlab.com/gitlab-org/gitlab-ce/issues/40287#note_67608034

/cc @nolith @ClemMakesApps @annabeldunstone

What are the relevant issue numbers?

Related to https://gitlab.com/gitlab-org/gitlab-ce/issues/40287

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• Tests added for this feature/bug
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides