Add a document on query performance trade-offs

[mglukhovsky]

Some queries can be rewritten in different ways to take advantage of parallelization or to optimize performance (e.g. reordering terms can change the latency / performance of a query).

A good example of this is rethinkdb/rethinkdb#3756.

Since we don't have an optimizer, it would be good to document some of these details on a case-by-case basis for advanced users who are trying to understand why some queries are slower / faster.