doc: Warn against `uncaughtException` dependency.

State in the documentation that `uncaughtException` is not a reliable
way to restart a crashed application, and clarify that an application
may crash in ways that do not trigger this event.

Use a documented synchronous function in example code.

Fixes: https://github.com/nodejs/node/issues/6223



Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
PR-URL: https://github.com/nodejs/node/pull/6378