src: deprecate undocumented variables

The `root` and `GLOBAL` never be documented.

src/node.js

@@ -159,8 +159,28 @@
   startup.globalVariables = function() {
     global.process = process;
     global.global = global;
-    global.GLOBAL = global;
-    global.root = global;
+
+    // Deprecate GLOBAL and root
+    ['GLOBAL', 'root'].forEach(function (name) {
+      // getter
+      const get = NativeModule.require('util').deprecate(function() {
+        const desc = { configurable: true, enumerable: true, value: this };
+        Object.defineProperty(this, 'GLOBAL', desc);
+        return this;
+      }, "'" + name + "' is deprecated, use 'global'");
+      // setter
+      const set = function(value) {
+        const desc = { configurable: true, enumerable: true, value: value };
+        Object.defineProperty(this, name, desc);
+      };
+      // define property
+      Object.defineProperty(global, name, {
+        configurable: true,  // but not enumerable
+        get: get,
+        set: set
+      });
+    });
+
     global.Buffer = NativeModule.require('buffer').Buffer;
     process.domain = null;
     process._exiting = false;