This chapter of the Transactions Troubleshooting Guide provides a brief introduction for the high-level Transactions concepts.

Tracker to include topics for the chapter 'Introducing Transactions' based on the outline draft created here:
https://docs.google.com/document/d/10So5wUflZLvoYbQyc7aMNZeyNYFBZtG01p0BFuBhZy0/edit?usp=sharing