Create separate document describing resource usage and privacy in more detail

[jkarlin]

TAG reviewers request (w3ctag/design-reviews#95) a document to flesh out the privacy and resource usage issues of background tasks on the web. Both to enlighten users as well as to guide implementers for background sync as well as other specifications that involve background operations.

Topics might include:

1. More thorough discussion of privacy and resource risks
2. Mitigations and thoughts to keep in mind while implementing such as:
   1. Capping the runtime of a background task
   2. Capping the number of retried background tasks
   3. Preventing background tasks from being able to run forever (e.g., don't let them reschedule themselves in the background and don't let them run sequentially)