Clarify definition of Capacity in the spec

[j-griffith]

There are some questions that have come up around the definition of capacity and how it differs between raw and file-system based provisioners. There may also be some misinterpretation on the impact attach/mkfs might have on this reported value.

It would be good to explicitly define these things in the spec so there's no confusion.

Just to expand on this here's what my assumption has been, and it seem they may not match with others thus they may be incorrect, or need clarification in the spec:

In the case of a block device:

• User requests a PVC with capacity of 1 Gig
• Storage backend provides a volume that will have a raw capacity of 1 Gig
• If formatted and attached to a container in a POD, the actually usable filesystem capacity will be less than the raw capacity requested/reported of 1 Gig.

[saad-ali]

Good point. Worth clarifying.