doc: mark global object as legacy

Author: mertcanaltin

doc/api/globals.md

@@ -509,6 +509,8 @@ added: v0.1.27
 
 <!-- type=global -->
 
+> Stability: 3 - Legacy. Use [`globalThis`][] instead.
+
 * {Object} The global namespace object.
 
 In browsers, the top-level scope is the global scope. This means that
@@ -1028,6 +1030,7 @@ A browser-compatible implementation of [`WritableStreamDefaultWriter`][].
 [`console`]: console.md
 [`exports`]: modules.md#exports
 [`fetch()`]: https://developer.mozilla.org/en-US/docs/Web/API/fetch
+[`globalThis`]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/globalThis
 [`module`]: modules.md#module
 [`perf_hooks.performance`]: perf_hooks.md#perf_hooksperformance
 [`process.nextTick()`]: process.md#processnexttickcallback-args