docs: add note about using cluster networking-less

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.

Refs: https://github.com/nodejs/help/issues/970

Checklist

• documentation is changed or added
• commit message follows commit guidelines

Affected core subsystem(s)

doc