doc: add documentation deprecation for process._tickCallback

This change also supports --pending-deprecation for the new deprecation.

PR-URL: https://github.com/nodejs/node/pull/29781
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>

doc/api/deprecations.md

@@ -2519,6 +2519,19 @@ Type: Documentation-only
 Prefer [`response.socket`][] over [`response.connection`] and
 [`request.socket`][] over [`request.connection`].
 
+<a id="DEP0XXX"></a>
+### DEP0XXX: process._tickCallback
+<!-- YAML
+changes:
+  - version: REPLACEME
+    pr-url: https://github.com/nodejs/node/pull/29781
+    description: Documentation-only deprecation.
+-->
+Type: Documentation-only (supports [`--pending-deprecation`][])
+
+The `process._tickCallback` property was never documented as
+an officially supported API.
+
 [`--pending-deprecation`]: cli.html#cli_pending_deprecation
 [`--throw-deprecation`]: cli.html#cli_throw_deprecation
 [`Buffer.allocUnsafeSlow(size)`]: buffer.html#buffer_class_method_buffer_allocunsafeslow_size

lib/internal/bootstrap/pre_execution.js

@@ -271,6 +271,10 @@ function initializeDeprecations() {
     process.binding = deprecate(process.binding,
                                 'process.binding() is deprecated. ' +
                                 'Please use public APIs instead.', 'DEP0111');
+
+    process._tickCallback = deprecate(process._tickCallback,
+                                      'process._tickCallback() is deprecated',
+                                      'DEP0XXX');
   }
 
   // Create global.process and global.Buffer as getters so that we have a