Documentation of secrets doesn't match code

[Tam-Lin]

The docs say (https://docs.chaostoolkit.org/reference/api/experiment/#action, for example) that actions and probes must declare a type, name, provider, and so on, and may declare secrets. However, that doesn't work; secrets have to be declared a property of the provider in order to work.

Opening this to document it; I'll try to push a change to the docs at some point, but I'm not sure how much in there needs to change.