doc: don't recommend domains for error handling

Remove the suggestion to use domains for exception handling. Add clarity
to "unhandledException".

Fixes: https://github.com/nodejs/io.js/issues/2055
PR-URL: https://github.com/nodejs/io.js/pull/2056


Reviewed-By: Trev Norris <trev.norris@gmail.com>
Reviewed-By: Chris Dickinson <chris@neversaw.us>