doc: more use-cases for promise events

This adds an example of a semi-common promise pattern that could
result in false-positive 'unhandledRejection' events, to help motivate
why there is a dual 'rejectionHandled' event. I anticipate it being
helpful to users who encounter 'unhandledRejection' events that do not
stem from obvious typos such as the JSON.pasre example.

Also cleans up the promise rejection tracking sample. By using a Map
instead of array we can get O(1) deletion and actually record the errors.
And, fixed indentation and backtick usage there to align with the rest of
the document.

Reviewed-By: Stephan Belanger <admin@stephenbelanger.com>
Reviewed-By: Petka Antonov <petka.antonov@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
PR-URL: https://github.com/nodejs/node/pull/3438