doc: mark accessing IPC channel fd as undefined

Adds note that accessing the fd of the IPC channel in any other way
than process.send, or using the IPC channel with child processes that
is not Node.js is not supported.

PR-URL: https://github.com/nodejs/node/pull/17545


Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>