Document related resources relationship authorization

[valscion]

We should document these two cases in https://github.com/venuu/jsonapi-authorization/blob/master/docs/relationship-authorization.md:

• GET /source_record/:id/has-one-resource
• GET /source_record/:id/has-many-resources

Look at the existing docs in that file as an example on how these specs should be written.

Originally posted by @valscion in https://github.com/venuu/jsonapi-authorization/pull/113#issuecomment-468296430

It is not mentioned in the docs, only the create/update/delete behaviour of relationships is in the docs, but that is a different case.

Oh wow, that seems like an oversight. I'll create an issue about this, as we will likely want to document this in https://github.com/venuu/jsonapi-authorization/blob/master/docs/relationship-authorization.md