doc: clarify the use of `option.detached`

This paragraph conveys that detached child processes do not stay
running in the background in general. Instead clarify that this
refers to the parent process exiting before the detached child
process is complete.

Reviewed-By: Trevor Norris <trev.norris@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
PR-URL: https://github.com/nodejs/node/pull/3250