docs: Add note about global scope

doc/api/globals.markdown

@@ -6,6 +6,11 @@ These object are available in the global scope and can be accessed from anywhere
 
 The global namespace object.
 
+In browsers, the top-level scope is the global scope. That means that in
+browsers if you're in the global scope `var something` will define a global
+variable. In Node this is different. The top-level scope is not the global
+scope; `var something` inside a Node module will be local to that module.
+
 ### process
 
 The process object. See the `'process object'` section.