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>

doc/api/deprecations.md

@@ -2416,6 +2416,21 @@ also have an `index.js` file in the top level directory will resolve the
 `index.js` file. That is deprecated and is going to throw an error in future
 Node.js versions.
 
+<a id="DEP0XXX"></a>
+### DEP0XXX: ChildProcess._channel
+<!-- YAML
+changes:
+  - version: REPLACEME
+    pr-url: https://github.com/nodejs/node/pull/26982
+    description: Documentation-only.
+-->
+
+Type: Documentation-only
+
+The `_channel` property of child process objects returned by `spawn()` and
+similar functions is not intended for public use. Use `ChildProcess.channel`
+instead.
+
 [`--pending-deprecation`]: cli.html#cli_pending_deprecation
 [`Buffer.allocUnsafeSlow(size)`]: buffer.html#buffer_class_method_buffer_allocunsafeslow_size
 [`Buffer.from(array)`]: buffer.html#buffer_class_method_buffer_from_array