Define what the 2 policy-controlled features actually control

[miketaylr]

Right now https://wicg.github.io/local-network-access/#integration-with-permissions-policy just says that "local-network" and "loopback-network" exist, but doesn't say what they control, or what happens if a document isn not allowed to use them.

Here's a few examples of what fixing this might look like:

1. https://wicg.github.io/ua-client-hints/#permisisons-policy-integration and step 6 of https://wicg.github.io/ua-client-hints/#getHighEntropyValues
2. https://w3c-fedid.github.io/FedCM/#permissions-policy-integration and step 3 of https://w3c-fedid.github.io/FedCM/#browser-api-identity-credential-disconnect

[christhompson]

The PR in #59 will add the checks for "allowed to use", but was in limbo pending resolving how we were going to split apart the permission. I believe that once that is updated using the new split feature names, this should resolve this, but let me know if I'm missing something.

[miketaylr]

Ah, excellent - I should have looked in pending PRs. :)