Clarify `description` vs `metadata` before next release

[richmolj]

The catalog capability in working-draft introduces metadata described as:

Business-defined custom data extending the standard product model.

We also have description for a similar purpose e.g. providing a shipping date range (since there are no structured date fields in the core Fulfillment extension).

These are both important, powerful concepts so we should clarify their usage and make first-class spec citizens. My sense is:

• description: presentational; buyer-facing display content.
  • Example: display a shipping date range for when the package arrives.
• metadata: business-defined extensibility data - it lets merchants attach custom structured data that the
  standard product model doesn't cover. It's not necessarily shown to the buyer; it's for the business/system
  to extend the schema.
  • Example: structured data that feeds into search engine algorithms

Side note: I've found it helpful to describe UCP as an Experience API, not a System API. Not everything needs to be structured, documented, typed, comprehensive. May be worth framing the spec this way, YMMV.