docs: add examples for getters in other namespaced modules (#1800)

docs/guide/modules.md

@@ -151,6 +151,7 @@ modules: {
       someGetter (state, getters, rootState, rootGetters) {
         getters.someOtherGetter // -> 'foo/someOtherGetter'
         rootGetters.someOtherGetter // -> 'someOtherGetter'
+        rootGetters['bar/someOtherGetter'] // -> 'bar/someOtherGetter'
       },
       someOtherGetter: state => { ... }
     },
@@ -161,6 +162,7 @@ modules: {
       someAction ({ dispatch, commit, getters, rootGetters }) {
         getters.someGetter // -> 'foo/someGetter'
         rootGetters.someGetter // -> 'someGetter'
+        rootGetters['bar/someGetter'] // -> 'bar/someGetter'
 
         dispatch('someOtherAction') // -> 'foo/someOtherAction'
         dispatch('someOtherAction', null, { root: true }) // -> 'someOtherAction'