Primer: Explain different bases on which IRI expansion occurs

[ajs6f]

IRIs can be expanded in a few ways depending on their use (e.g. as value or as vocabulary). This is a more-or-less unavoidable inheritance from JSON, but it can be unexpected for some users.

We should give the most clear and straightforward explanation we can of the different bases on which expansion can occur, and how to control each insofar as that is possible.