Standardize code snippets in package-info

[ajkannan]

Right now, the example in the Datastore's and Storage's package-info.java files explicitly sets the service options' project IDs but not the auth credentials. The user will probably specify both or none, depending on whether they run the code from App/Compute Engine or not.

I think we should have two examples in each package info, one with both left unset and the other with both explicitly set.