Document `useStore()`

Signed-off-by: Phillip Rak <rak.phillip@gmail.com>

shell/composables/useStore.ts

@@ -1,5 +1,11 @@
 import { getCurrentInstance } from 'vue';
 
+/**
+ * useStore allows for accessing Vuex stores from within a setup function. This is a temporary measure for working with
+ * Vuex in Vue2.
+ *
+ * TODO: #9541 Remove for Vue 3 migration
+ */
 export const useStore = ():unknown => {
   const vm = getCurrentInstance();