Add join docs.
Add section on caching results.
Add example of how to build cached results in 2 different ways.
Build out more sections in the query builder docs.
Add section for expression builder.
Clean up and clarify expression docs.

* Be more squishy with generated SQL. It will differ because of bound
  parameters.
* Try to clarify section.
Update docs for unionAll.
Add more examples of how to build queries.

Show how to use where(), andWhere() and orWhere() better.
Start queries with where().
Merge pull request #963 from cakephp/3.0-query-pt2

3.0 Query builder part 2