More explicitly document use cases

[equalsJeffH]

We need to provide more specific guidance for RPs about how to configure the several options for navigator.credentials.{create,get} and, secondly, to provide a reserve mapping of options back to intents.

See PR #1300

[equalsJeffH]

@agl summarily closed PR #1300, with the nominal suggestion to simply cite https://github.com/balfanz/how-to-fido (i.e., whenever/wherever it morphs into a more "official" document). We certainly ought to cite the latter. However, though, readers may suffer impedance mismatches between what is presently in the WebAuthn spec and the latter document unless the present webauthn scenarios section(s) are appropriately massaged (at least some?) --- we should probably at least take a look and see if we feel that may be the case.

In any case, I've removed the pr-open label from this issue for the time being.

[equalsJeffH]

i think we should do something along these lines, but it can be addressed in a milestone later than wd-03