Need more streaming documentation

[Chris Hillery]

Zorba supports streaming data in some cases, which allows querying arbitrarily-sized input data without exploding memory requirements. However, we have very little documentation about what functions support this, and none at all explaining how you can use this "safely". It's very easy to make a small change to a query that breaks streaming; for example, see

  https://groups.google.com/forum/#!topic/zorba-dev/JMdPkBhXSMM

We should introduce a separate page in our doc (and publicize it with a blog entry) describing how this works and how to use it well.