doc: deprecate notice for process methods

Documentation of deprecation notice `_getActiveRequestsInfo` and
`_getActiveHandlesInfo` since they are internal usage and can be removed
in future releases.

doc/api/deprecations.md

@@ -3056,6 +3056,23 @@ This method was deprecated because it is not compatible with
 
 Use [`buffer.subarray`][] which does the same thing instead.
 
+### DEP0159: Deprecated `_getActiveRequestsInfo` and `_getActiveHandlesInfo`
+
+<!-- YAML
+changes:
+  - version: REPLACEME
+    pr-url: https://github.com/nodejs/node/pull/41587
+    description: Documentation-only deprecation.
+-->
+
+Type: Documentation-only
+
+The `process._getActiveHandlesInfo()` and `process._getActiveRequestsInfo()`
+is not intended for public use and can be removed in future releases.
+
+Use `process.getActiveResourcesInfo()` to get the list of handles and not
+the actual references.
+
 [Legacy URL API]: url.md#legacy-url-api
 [NIST SP 800-38D]: https://nvlpubs.nist.gov/nistpubs/Legacy/SP/nistspecialpublication800-38d.pdf
 [RFC 6066]: https://tools.ietf.org/html/rfc6066#section-3