Clarify value types for granularities; link to more ISO info; more info on OrderByColumnSpec

And those minor edits are:

* Clarified that Duration granularity values are Java longs
* Link to Wikipedia ISO8601 page for those who'd like to read up on that standard to make sense of some of the examples and how to create their own
The key point seems to be that duration granularity is a String whose value must be an "integer".