Update status of registry spec for Tech Preview

[aphillips]

@macchiati suggested in #671

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In the Conformance and Use:

The term 'registry' is being used ambiguously. That is, the distinction is not clearly enough drawn between the default registry, and the machine-readable format that is used by the default registry, but can also be used as implementations to provide machine-readable documentation/specification of their own registry, with additions to the default registry.

For example, "The MessageFormat 2.0 Registry was created to describe the core set of formatting and selection functions," seems really to be describing the default registry, not the registry format. So it should say "The MessageFormat 2.0 Default Registry"

In the next paragraph "The registry provides a machine-readable description" seems to be talking about the use as a format. It should have its own term, like 'registry format' (but doesn't have to be exactly that term).

I suggest that we don't have any 'naked' mentions of the word 'registry' in the spec, but rather qualify it to resolve the ambiguity.

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I made other comments there. This issue tracks making registry terms consistent and adding a note for tech preview