target-level caching as a service design: add clarifying comments

- Document that `just serve` can be asked to back up a tree to the
  remote execution end point.
- Specify, that similar to getting the tree of a commit, just serve
  can also be asked to provide the tree of an unpacked archive
  given by its blob identifier.
- Document the auxilliary request by which `just build` can get
  the needed information about the flexible variables of an export
  target in order to construct the correct cache key.

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