doc: request to expand on process "exit" event #2924

Reference: #2918 (comment)

The process "exit" event has a lot of reasons why it will or won't be emitted. Unfortunately, these reasons are not exactly clear from the documentation. That makes it a rather useless event, unless each user goes through a painful, time-consuming trial-and-error phase to figure out how things work (in the worst case, people will find out in production). It also doesn't set a guideline for developers as to how things are supposed to work (sure, we should have unit tests to confirm things work the way they do, but still).

cc @Trott