Document revocation action Python dictionary

[axelsimon]

The Secure Payloads doc (text in review here) mentions:

Revocation actions are small Python scripts that will run on an Agent when a valid revocation is received. They should contain an execute function that takes one argument. This argument is a Python dictionary of metadata that can be used to tailor what the revocation action does.

This dictionary of actions should be documented somewhere, as pointed out by @mpeters.

I suggest the Knowers™ to answer here (@jetwhiz, @nabilschear, @lukehinds ?) and then we can turn that into proper documentation.

Thanks!