Add section on operation granularity

[markweitzel]

Based on a review comment from Krzysztof: "It's possible that I missed it, but I think we should have a section on granularity of service operations. From time to time we run into cases where a single user scenario corresponds to too many service calls. This means that client library has to to many calls to the service to actually do what the user cares about." This would make a good section in the considerations document.