There's currently no documentation in docs/ on Synapse's Third Party Event Rules. Ideally the documentation would have:

• An explanation of what it is and why it exists (the sample config does an ok job)
• How to configure it (again, sample config)
• What methods are available to be overridden, and which are optional and required (one can currently figure this out by looking at the class itself
• Any examples of implementations (I believe DINUM's one is currently the only known example)

All following a similar format to the Password Auth Providers documentation.