This patch completes the api ref for standard attribute based revision
numbers by:
- Updating applicable resource JSON responses to include
revision_number.
- Documents the revision_number response param where missing.
- Adds a section in the intro about revisions as well as conditional
HTTP based operations using If-Match header.
- Standardizes the revision_number response param; doesn't seem
necessary for each resource to have it's own.
Change-Id: Ib9a19126c35f89bc9c2ad891a7339702a406e2d5
Partial-Bug: #1703234
41212a5311