Document how tests are executed (closed vs open system model, VUs etc.)

[robingustafsson]

We need to do a better job of explaining how k6 tests are executed, the difference between closed and open system models [1], and how VUs, duration and iterations relate to each other.

We have a start in [2], but we can improve it. Also, this relates to #550.

[1] - http://users.cms.caltech.edu/~adamw/publications-openclosed.html
[2] - https://docs.k6.io/docs/running-k6#section-adding-more-vus

[na--]

This was done in grafana/k6-docs#49