Clarify documentation about bootstrapping and syncing core packages.

[bdausses]

Documentation about getting and maintaining core packages is somewhat confusing. Ideally, we should call this out as a two step process. I guess I see this that way, anyways...

1.) Bootstrap is a one time process to get the bulk of the core origin down.
2.) Sync is something that would ideally be happening in an automated fashion on some sort of a schedule to keep the on-prem core packages in sync with what is available on the SAAS builder.