Clean up coherence chapter in the Chalk book

[seanchen1991]

From the preamble of the chapter in the Chalk book on coherence:

This document was previously prepared for the initial design of coherence rules in Chalk. It was copy-pasted here on 2020-10-06, but has not been vetted for accuracy of the current implementation or edited for clarity.

This section should be edited for correctness and cleaned up to match the other chapters in the book.

[seanchen1991]

@rustbot claim