Context

`just -l` in a PyMongo clone will list the available recipes. In the `[test]` group a brief explanation of what each recipe would be helpful and potentially avoid a trip to CONTRIBUTING.md for more information. 

Definition of done

Add documentation comments to the justfile for the test group and possibly other groups.

Pitfalls

A trip to the CONTRIBUTING.md file may still be necessary to fully undestand the justfile's capability.