docs: update vue-router documentation (#1701)

Update documentation to explain differences between using `localVue` and `mocks` and provide examples of the use case for each.

A common gotcha with integration testing (which don't stub children) is when using the `mocks` approach the router instance is not available on child components.

Author: blake-newman

docs/guides/using-with-vue-router.md

@@ -41,17 +41,20 @@ shallowMount(Component, {
 ### Installing Vue Router with localVue
 
 ```js
-import { shallowMount, createLocalVue } from '@vue/test-utils'
+import { mount, createLocalVue } from '@vue/test-utils'
 import VueRouter from 'vue-router'
 
 const localVue = createLocalVue()
 localVue.use(VueRouter)
 
-shallowMount(Component, {
-  localVue
+mount(Component, {
+  localVue,
+  router
 })
 ```
 
+The router instance is available to all children components, this is useful for integration level testing.
+
 ### Mocking `$route` and `$router`
 
 Sometimes you want to test that a component does something with parameters from the `$route` and `$router` objects. To do that, you can pass custom mocks to the Vue instance.
@@ -72,6 +75,8 @@ const wrapper = shallowMount(Component, {
 wrapper.vm.$route.path // /some/path
 ```
 
+> **Note:** the mocked `$route` and `$router` values are not available to children components, either stub this components or use the `localVue` method.
+
 ### Common gotchas
 
 Installing Vue Router adds `$route` and `$router` as read-only properties on Vue prototype.