docs: component instance (#466)

docs/guide/advanced/component-instance.md

@@ -1,5 +1,78 @@
 # Component Instance
 
-Mostly `findComponent()`, `.props()` et al.
+[`mount`](/api/#mount) returns a `VueWrapper` with lots of convenient methods for testing Vue components. Sometimes you might want access to the underlying Vue instance. You can access that with the `vm` property.
 
-Also why `.vm` is not available and yet another recommendation to test outputs instead of implementation details.
+## A Simple Example
+
+Here is a simple component that combines props and data to render a greeting:
+
+```ts
+test('renders a greeting', () => {
+  const Comp = {
+    template: `<div>{{ msg1 }} {{ msg2 }}</div>`,
+    props: ['msg1'],
+    data() {
+      return {
+        msg2: 'world'
+      }
+    }
+  }
+
+  const wrapper = mount(Comp, {
+    props: {
+      msg1: 'hello'
+    }
+  })
+
+  expect(wrapper.html()).toContain('hello world')
+})
+```
+
+Let's take a look at what's available on `vm` by with `console.log(wrapper.vm)`:
+
+```js
+{
+  msg1: [Getter/Setter],
+  msg2: [Getter/Setter],
+  hasOwnProperty: [Function]
+}
+```
+
+We can see both `msg1` and `msg2`! Things like `methods` and `computed` properties will show up too, if they are defined. When writing a test, while it's generally recommended to assert against the DOM (using something like `wrapper.html()`), in some rare circumstances you might need access to the underlying Vue instance. 
+
+## Usage with `getComponent` and `findComponent`
+
+`getComponent` and `findComponent` return a `VueWrapper` - much like the one get from `mount`. This means you can also access all the same properties, including `vm`, on the result of `getComponent` or `findComponent`.
+
+Here's a simple example:
+
+```js
+test('asserts correct props are passed', () => {
+  const Foo = {
+    props: ['msg'],
+    template: `<div>{{ msg }}</div>`
+  }
+
+  const Comp = {
+    components: { Foo },
+    template: `<div><foo msg="hello world" /></div>`
+  }
+
+  const wrapper = mount(Comp)
+
+  expect(wrapper.getComponent(Foo).vm.msg).toBe('hello world')
+  expect(wrapper.getComponent(Foo).props()).toEqual({ msg: 'hello world' })
+})
+```
+
+A more thorough way to test this would be asserting against the rendered content. Doing this means you asserts the correct prop is passed *and* rendered. 
+
+::: tip
+
+Note: if you are using a `<script setup>` component, `vm` will not be available. That's because `<script setup>` components are [closed by default](https://github.com/vuejs/rfcs/blob/script-setup-2/active-rfcs/0000-script-setup.md#closed-by-default). For these components, and in general, consider avoiding `vm` and asserting against the rendered markup.
+:::
+
+## Conclusion
+
+- use `vm` to access the internal Vue instance
+- `getComponent` and `findComponent` return a Vue wrapper. Those Vue instances are also available via `vm`