This is a follow up PR after merging the Projections PR. This is
a combination of lots of small things.

- Add documentation to properties of `MapEntry` so that they are
displayed
- Move map#project so that we maintain alphabetical order
- Add some corner case tests for projections and aggregations
- Used asserCountEqual on projection tests so that the tests will
be more durable, even if we add more items to map
- Add missing API documentation for projections
- Fix API documentation of projections around the return value documentations
- Add unit tests for the invalid projection inputs
- Make the projections code snippet simpler
- Add a code sample for projections