Clarify use cases, requirements and constraints

[kinu]

From TAG review comments on 2014-02-04:

• Use case: React to space pressure in the environment to delete caches or other unnecessary data
• Use case: Ensure enough space will be available to store the result of a potentially-expensive computation or download, before performing that computation or download
• Requirement: Be able to measure the space taken up by various storage artifacts
• Requirement: Allow applications to choose the relative importance of keeping storage artifacts without regard for their storage medium (i.e. allow keeping important data in IndexedDB or unimportant caches in the filesystem)
• Requirement: Give apps enough information to notify the user about evicted storage artifacts (e.g. removing the "downloaded" checkmark from a video that was removed in response to space pressure)
• Constraint: Minimize user interaction ("infobars") except when unavoidable, e.g. for resolving a conflict between space pressure and an app's desire to keep a certain storage artifact inviolate
• Constraint: Any APIs specified must be flexible enough that it is clear how to extend them to future storage types, e.g. ServiceWorker caches

(TODO: these requirements/constraints should be probably filed as separate issues)

[kinu]

We're moving to https://github.com/w3c/quota-api/issues, closing some obsolete issues.