Add an "anatomy of a review" doc?

[jmacdotorg]

CodeRabbit's comments can be long, rich, and complex, with many sections. Some of the sections are collapsed, and it's not always clear by name alone what they do. It's also not always clear why CodeRabbit sometimes includes certain sections, or not.

We should document all the sections that might appear in a code review, the conditions under which CodeRabbit makes them, how CodeRabbit generates them, and what they all mean. That would do a lot to increase the transparency of this toolkit!

[linear]

ENG-1501 Add an "anatomy of a review" doc?

[jmacdotorg]

#363 might be a sub-task of this one.