child_process: doc deprecate ChildProcess._channel

This was eligible to begin deprecation in Node 8.

PR-URL: https://github.com/nodejs/node/pull/26982


Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: Sam Roberts <vieuxtech@gmail.com>