Add example for persisting api reducer

if you're using redux-persist by persisting individual reducers, you'll have multiple `REHYDRATE` actions and will need to respond to the correct one. Added a code example

Author: evertbouw

docs/rtk-query/usage/persistence-and-rehydration.mdx

@@ -44,9 +44,15 @@ export const api = createApi({
   baseQuery: fetchBaseQuery({ baseUrl: '/' }),
   // highlight-start
   extractRehydrationInfo(action, { reducerPath }) {
+    // when persisting the root reducer
     if (action.type === REHYDRATE) {
       return action.payload[reducerPath]
     }
+
+    // when persisting the api reducer
+    if (action.type === REHYDRATE && action.key === 'key used with redux-persist') {
+      return action.payload
+    }
   },
   // highlight-end
   endpoints: (build) => ({