BatteryManager: Describe instance creation in an algorithm

[rakuco]

This came up in #46 (specifically, here). @marcoscaceres says:

yeah, let's do that as followup... fwiw, I'm imagining this prose need to go into some initializer steps... as an example:
https://w3c.github.io/geolocation-api/#constructing-a-geolocationposition

Right now it is not very clear what should happen when a new BatteryManager is created: when exactly do the internal slots have their initial values set (vs when do we start checking)? When do we send the first event?