Preface each executor with an example use case

[MattDodsonEnglish]

Right now, the executor exercises only describe what the function does, but they don't describe context.

For example, I'm still unclear about when a user would prefer to configure VUs vs arrival rate. To me, arrival rate doesn't seem to have any real value other than that people are used to configuring RPS. I am probably wrong about this, because I don't know day-to-day load testing operations and I lack imagination.

It'd be great if the exercises were framed in a more concrete context.

For what it's worth, the docs aren't any better on this point. For example:

Use when
Use this executor if you need a specific amount of VUs to complete the same amount of iterations. This can be useful when you have fixed sets of test data that you want to partition between VUs.

What I'm thinking is Ok. And when should I do that?

This issue ties in well with grafana/k6-docs#808.

[javaducky]

I lack imagination

🤣