Clarify job selection algorithm docs

What does this MR do?

Clarifies the language in the documentation on runners' job selection algorithm.

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

Just to make sure this reads more clearly than before!

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

N/A