docs(guide): note about slot scope when not wrapped in template tag

https://github.com/vuejs/test-utils/blob/39d86ad5abe71bac8d76c5b82f11f30225cf8673/src/utils/compileSlots.ts#L10

The only example provided used an explicit #default="params", but it doesn't mention that if you don't explicitly wrap it, the slot scope becomes also available as params. In VTU1, they were exposed as props, so i think it's better to be explicit about it.

docs/guide/advanced/slots.md

@@ -139,7 +139,7 @@ test('layout full page layout', () => {
 
 ## Scoped Slots
 
-[Scoped slots](https://v3.vuejs.org/guide/component-slots.html#scoped-slots) and bindings are also supported.
+[Scoped slots](https://v3.vuejs.org/guide/component-slots.html#scoped-slots) and bindings are also supported. 
 
 ```js
 const ComponentWithSlots = {
@@ -158,8 +158,8 @@ const ComponentWithSlots = {
 test('scoped slots', () => {
   const wrapper = mount(ComponentWithSlots, {
     slots: {
-      scoped: `<template #scoped="params">
-        Hello {{ params.msg }}
+      scoped: `<template #scoped="scope">
+        Hello {{ scope.msg }}
         </template>
       `
     }
@@ -169,6 +169,19 @@ test('scoped slots', () => {
 })
 ```
 
+When using string templates for slot content, **if not explicitly defined using a wrapping `<template #scoped="scopeVar">` tag**, slot scope becomes available as a `params` object when the slot is evaluated.
+
+```js
+test('scoped slots', () => {
+  const wrapper = mount(ComponentWithSlots, {
+    slots: {
+      scoped: `Hello {{ params.msg }}` // no wrapping template tag provided, slot scope exposed as "params"
+    }
+  })
+
+  expect(wrapper.html()).toContain('Hello world')
+})
+
 ## Conclusion
 
 - Use the `slots` mounting option to test components using `<slot>` are rendering content correctly.