Update headers slightly for API specification(s)

Update the headers slightly for API specifications to make the sections
a bit easier to read. The V3 and V2.0 specifications do not need to be
top-level and can easily be nested under a more generic section.

Change-Id: Id636ce72460423597a6f11b24530082b8ae9ddc8
Merge "Update headers slightly for API specification(s)"