Clarify in ZIP 0 and/or the ZIP Guide how Deployment sections should be written

[str4d]

In particular, the Deployment section should end up being a statement about how / when the Specification was deployed, rather than being a conformance requirement on how it must be deployed.

If a ZIP has specific deployment requirements, those should be given in its Specification section (or in the equivalent section of the NU deployment ZIP, e.g. if two ZIPs have a deployment interaction).

Older ZIPs don't necessarily follow this pattern (because we were still figuring out how to write ZIPs). We might want to clean them up at some point.