doc: add note about using cluster without networking

Although the primary use-case for the cluster module is networking, the
module provides a generic master/worker interface that could also be
used if you dont use networking at all. Currently the docs are a bit
ambiguous about this as only the primary use-case is ever mentioned,
this remark should clarify that the cluster module can also be used
without disadvantages if you dont use networking.

PR-URL: https://github.com/nodejs/node/pull/17031
Refs: https://github.com/nodejs/help/issues/970


Reviewed-By: Gireesh Punathil <gpunathi@in.ibm.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>