Clean up identifier definition

[jandrieu]

The VCDM currently defines the id property as a single URL:

id
The id property is OPTIONAL. If present, id property's value MUST be a single URL, which MAY be dereferenceable. It is RECOMMENDED that the URL in the id be one which, if dereferenceable, results in a document containing machine-readable information about the id.

However, in the paragraph above, we state explicitly that ids include UUIDs such as urns:

Example id values include UUIDs (urn:uuid:0c07c1ce-57cb-41af-bef2-1b932b986873), HTTP URLs (https://id.example/things#123), and DIDs (did:example:1234abcd).

And we define URLs as dereferenceable:

URL
A Uniform Resource Locator, as defined by the URL Standard. URLs can be dereferenced to result in a resource, such as a document. The rules for dereferencing, or fetching, a URL are defined by the URL scheme. This specification does not use the term URI or IRI because those terms have been deemed to be confusing to Web developers.

Unfortunately, while the WHATWG spec does allow URNs (any URI scheme is considered a URL), the statement that "URLs can be dereferenced to result in a resource, such as a document." is inaccurate. We make the point in the definition of id that it MAY be referenceable, but in the URL definition we muddy the waters.