Mention possible consequence of diverging values

What does this MR do?

Adds a more concrete example of what can happen when not following the advice – only saying "more predictable results" was very abstract. Also clarifies not defining min_concurrency.

We ran into this while working on 🎫 #358871 (internal). The exact circumstances were a bit more specific:

• min_concurrency not defined at all, another detail this MR aims to explain better
• routing_rules set up in a certain (but not uncommon) way

Considering this entire topic is already deprecated (as far as I understood things), this feels like a boring solution that improves the status quo – while we could add even more details based on the case above, it would further complicate an already fairly complex section.

Related issues

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