enhancement: docs: make instructions for getting example creds more clear

[kbroughton]

I was able to create creds with
az ad sp create-for-rbac --name myServicePrincipal --password mypwd

Though it wasn't obvious how to map it to the credentials.
I eventually found that this works:

AZURE_CLIENT_ID=appId
AZURE_CLIENT_OID=principalId
AZURE_CLIENT_SECRET=mypwd

I had to run az role assignment list --assignee
to get the principalId and use that as the OID.
You might want to add that to the documentation.