The coroutines guide should mention happens-before guarantees

[dkhalanskyjb]

There are happens-before guarantees for executing the same coroutine even when it changes threads: https://proandroiddev.com/what-is-concurrent-access-to-mutable-state-f386e5cb8292 We occasionally get asked about this, and https://kotlinlang.org/docs/shared-mutable-state-and-concurrency.html has no mention of happens-before.