Add transition as root to the migration guide

src/.vuepress/config.js

@@ -202,6 +202,7 @@ const sidebar = {
         '/guide/migration/slots-unification',
         '/guide/migration/suspense',
         '/guide/migration/transition',
+        '/guide/migration/transition-as-root',
         '/guide/migration/transition-group',
         '/guide/migration/v-on-native-modifier-removed',
         '/guide/migration/v-model',

src/guide/migration/transition-as-root.md

@@ -0,0 +1,61 @@
+---
+badges:
+  - breaking
+---
+
+# Transition as Root <MigrationBadges :badges="$frontmatter.badges" />
+
+## Overview
+
+Using a `<transition>` as a component's root will no longer trigger transitions when the component is toggled from the outside.
+
+## 2.x Behavior
+
+In Vue 2, it was possible to trigger a transition from outside a component by using a `<transition>` as the component's root:
+
+```html
+<!-- modal component -->
+<template>
+  <transition>
+    <div class="modal"><slot/></div>
+  </transition>
+</template>
+```
+
+```html
+<!-- usage -->
+<modal v-if="showModal">hello</modal>
+```
+
+Toggling the value of `showModal` would trigger a transition inside the modal component.
+
+This worked by accident, not by design. A `<transition>` is supposed to be triggered by changes to its children, not by toggling the `<transition>` itself.
+
+This quirk has now been removed.
+
+## Migration Strategy
+
+A similar effect can be achieved by passing a prop to the component instead:
+
+```vue
+<template>
+  <transition>
+    <div v-if="show" class="modal"><slot/></div>
+  </transition>
+</template>
+<script>
+export default {
+  props: ['show']
+}
+</script>
+```
+
+```html
+<!-- usage -->
+<modal :show="showModal">hello</modal>
+```
+
+## See also
+
+- [Some transition classes got a rename](/guide/migration/transition.html)
+- [`<TransitionGroup>` now renders no wrapper element by default](/guide/migration/transition-group.html)

src/guide/migration/transition-group.md

@@ -38,3 +38,4 @@ In Vue 3, we have [fragment support](/guide/migration/fragments.html), so compon
 ## See also
 
 - [Some transition classes got a rename](/guide/migration/transition.html)
+- [`<Transition>` as a root can no longer be toggled from the outside](/guide/migration/transition-as-root.html)

src/guide/migration/transition.md

@@ -63,4 +63,5 @@ The `<transition>` component's related prop names are also changed:
 
 ## See also
 
+- [`<Transition>` as a root can no longer be toggled from the outside](/guide/migration/transition-as-root.html)
 - [`<TransitionGroup>` now renders no wrapper element by default](/guide/migration/transition-group.html)