Microsoft Azure released their usage API and have documented it here. As documented, they have released a new output format for the usage API. However, the output I receive doesn't seem to absolutely conform to how it has been documented:
As you can see, I receive the value
Microsoft.Resources, which has the other values nested in it, which isn't how the documentation says.
Is there any other reference document I can refer to, which is more exhaustive about the API output?
Unfortunately, the answer to your question is not really. The documentation for Azure does lag a long way behind their development cycle.
Which does bring special challenges to anyone trying to code against it. The best place to go to try to understand what is happening is the Azure Github Account. There is some specification documentation there (though it is lacking in places) and also some SDKs that you can pick through and try to find what you seek.