Bug: Spec is not clear on hybrid Attached & Detached crate best practice

[ptsefton]

Describe the bug

It may not be clear to implementers how to make crates on a server that have attached data for download as well as detached URLs for fetching the data.

URL

https://www.researchobject.org/ro-crate/1.2-DRAFT/data-entities.html#embedded-data-entities-that-are-also-on-the-web

Suggested fix Will make it clearer in the spec that if local files are present and described then the @id MUST be local path URI and downloads can be in contentUrl

Additional context

From a discussion on Slack re ROHub with @dgarijo and @rapw3k and @stain