fix(docs) Clear description of update server changes TBD

Pulling in @philips' changes from coreos/coreos-cloudinit#6 after
trashing PR coreos/coreos-cloudinit#20.  Cleanup of that PR was
beyond my git-fu.

cc @jonboulle
fix(doc) more sensible ordering

It makes a bit more sense to specify the scope of the section
before getting into details about how it's done.
Merge pull request #120 from brianredbeard/pr20-fix

fix(docs) Clear description of update server changes