Improve documentation about open vs closed

[rocketraman]

What?

The documentation on open vs closed models needed improvement. The introduction stated:

Different models suit different test aims

But had no discussion whatsoever of which test aims were suited for which model.

In addition, the discussion about the drawbacks of the closed model was incorrect. It stated:

This effect is not ideal when the goal is to simulate a certain arrival rate of new VUs, or more generally throughput (e.g. requests per second).

The second part of this sentence should be "latency", not "throughput". Its measuring latency that is affected by coordinated omission, as new requests wait for old ones to finish, and that waiting time is not included in the latency measurements. The closed model is actually useful for measuring throughput.

I've:

1. added a new section "Open vs Closed Model" to the end of this document,

2. moved the discussion of the drawbacks of the closed model to this new section, as it provides the necessary context and removed some duplication in the text,

3. fixed the wording of the drawbacks of the closed model,

4. added some notes about when one would prefer the closed model and when one would prefer the open model.

Checklist

Please fill in this template:

• [X] I have used a meaningful title for the PR.
• [X] I have described the changes I've made in the "What?" section above.
• [X] I have performed a self-review of my changes.
• [ ] ~I have run the make docs command locally and verified that the changes look good.~ Unable to do so as the command does not work.

Select one of these and delete the others:

If updating the documentation for the next release of k6:

• [X] I have made my changes in the docs/sources/next folder of the documentation.

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