Updating the spec for clarity and parity with the implementation of the V7 transaction details

[vivek-arte]

A quick summary of the changes here:

• The assetDescSize field in the issuance action description is encoded as a compactSize type, as is standard across the specification (and also allows for the use of the existing methods in the implementation).
• The description of the notes used inside the issuance bundle were explicitly serialized to remove ambiguity.
• The type of the valueBurn field was set to the standard uint64 type.
• The order of the Orchard bundle fields was adjusted to have the binding signature after the burn fields.

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[PaulLaux]

Very nice, added small comments