Various improvements (NixOS#15)

* Various improvements

- Remove unnecessary **Description** headers
- Rename **Rationale and Alternatives** to just **Alternatives**
- Insert must/may/should more diligently
- Add some TODOs where things are unclear
- Remove numberings from examples when not needed
- Minor clarity improvements and simplifications throughout

* Apply suggestions from code review

Co-authored-by: Ryan Hendrickson <ryan.hendrickson@alum.mit.edu>

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Co-authored-by: Ryan Hendrickson <ryan.hendrickson@alum.mit.edu>