docs: add more clarity for getters in a module usage (#1874)

close #1874

Author: kiaking

docs/guide/modules.md

@@ -85,7 +85,7 @@ const moduleA = {
 
 ## Namespacing
 
-By default, actions, mutations and getters inside modules are still registered under the **global namespace** - this allows multiple modules to react to the same mutation/action type.
+By default, actions and mutations are still registered under the **global namespace** - this allows multiple modules to react to the same action/mutation type. Getters are also registered in the global namespace by default. However, this currently has no functional purpose (it's as is to avoid breaking changes). You must be careful not to define two getters with the same name in different, non-namespaced modules, resulting in an error.
 
 If you want your modules to be more self-contained or reusable, you can mark it as namespaced with `namespaced: true`. When the module is registered, all of its getters, actions and mutations will be automatically namespaced based on the path the module is registered at. For example: