tweaks to YANG module ordering

[mcr]

> 28) Lines 624 and 609:  Probably both should say "RFC ZZZ", and then
> put a note to the RFC Editor to replace RFC ZZZ with the RFC number of
> this document when it is published.

> 29) Line 627:  I would probably remove this comment as it does not
> really add anything.

> 30) Line 629: Reusable groupings, like type definitions etc, should be
> defined before they are being used.  This makes the module definition
> cleaner and more easy to read.  Please move the definition of
> voucher-artifact-grouping before your top level statement.

> 31) 31a) Line 632: I don't understand this statement.  What do you
> mean  with "grouping defined for future augmentations"?

> 31b) If you want to allow for future extensions, it would be good to
> indicate what you have in mind, specifically.  Provide an example (not
> inside the data model definition, but in a subsequent section).