doc: document MODULE_NOT_FOUND error

Document `MODULE_NOT_FOUND` as a possible `code` for errors.

Also, some missorting of bottom references was fixed in passing.

PR-URL: https://github.com/nodejs/node/pull/21894
Fixes: https://github.com/nodejs/node/issues/21890


Reviewed-By: James M Snell <jasnell@gmail.com>