Rework definitions in "Quota" section / API steps for estimate

[inexorabletash]

The terms here seem confusing and/or restrictive, and even the references from estimate() seem incorrect ("origin's global quota" ???). Things like "on the hard drive" seem prescriptive and don't apply to many devices. Can we start this section from scratch, defining only what's needed?

Specifically, the definitions here just need to support what's in the API substeps for estimate(). I'd suggest renaming this section to e.g. rename 5 to "Usage and Quota" and define "usage of an origin" and "quota of an origin, and have the API steps call out to these definitions.