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>

doc/api/child_process.md

@@ -621,6 +621,10 @@ pipes between the parent and child. The value is one of the following:
    will enable [`process.send()`][], [`process.disconnect()`][],
    [`process.on('disconnect')`][], and [`process.on('message')`] within the
    child.
+
+   Accessing the IPC channel fd in any way other than [`process.send()`][]
+   or using the IPC channel with a child process that is not a Node.js instance
+   is not supported.
 3. `'ignore'` - Instructs Node.js to ignore the fd in the child. While Node.js
    will always open fds 0 - 2 for the processes it spawns, setting the fd to
    `'ignore'` will cause Node.js to open `/dev/null` and attach it to the