Updates based on RFC comments

[tomdonaldson]

This contains several small additions and clarifications based on RFC comments to date:

• Added MIVOT to the architecture diagram
• Added a sentence to clarify the (non)advice for ordering of ID and ref attributes.
• Adopted suggestion to calling out the equivalence of ET and TT timescale
• Clarified that MIVOT elements are allowed with the VOTable schema.
• Adopted suggested wording change about the use of type="meta" for a MIVOT RESOURCE.
• Corrected an attribute name in a LINK example in section A4. This makes the example schema compliant though there are other issues with the example that won't be fixed for v1.5.

[mbtaylor]

Looks fine apart from one comment.

[gmantele]

• Added a sentence to clarify the (non)advice for ordering of ID and ref attributes.

I am fine with the added clarification about this second point. Thank you 🙂