doc: process exit event is not guaranteed to fire

This change:

* notes that the exit event is not guaranteed to fire
* provides an example situation where the exit event may not fire
* makes a minor copyediting change
* enforces 80 character wrap in one place where it was not honored

Fixes: https://github.com/nodejs/node/issues/2853
PR-URL: https://github.com/nodejs/node/pull/2861


Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>