Document difference between arrays and streams

[danielcompton]

There is a difference between arrays and streams that are returned. order-by with a non-indexed field will return an array which has been eagerly loaded. Streams are lazily transformed. This can be a bit confusing for users, and should be documented somewhere.

Relates to: https://github.com/rethinkdb/rethinkdb/issues/4530 https://github.com/rethinkdb/rethinkdb/issues/2303 https://github.com/rethinkdb/rethinkdb/issues/4115