Add explanation of model output to documentation

[clizbe]

What and Why

We need to specify exactly what the numbers mean.

Possible Drawbacks

No response

Related Issues

Came up in #295

[clizbe]

Probably comes after #115

[clizbe]

First version now, then update after #115.

[datejada]

We are going to refactor the outputs, see discussion #688

So, it is better to pick this issue after the outcome of the discussion.