Add help text to runner edit page that tags should be separated by commas.

What does this MR do?

Fixes #37405 (closed).

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

Not sure if the intention was to just add the helper text or do true "tag as you type". If the later, should update the issue (a) to be clear (b) to include other places there are system tags (e.g. Project edit, etc.)

Why was this MR needed?

To fix #37405 (closed).

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests added for this feature/bug
• Review
  • Has been reviewed by UX
  • Has been reviewed by Frontend
  • Has been reviewed by Backend
  • Has been reviewed by Database
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together

What are the relevant issue numbers?

Closes #37405 (closed)