docs(api): add `<ClientOnly>` to API docs (#8400)

docs/content/3.api/2.components/1.client-only.md

@@ -0,0 +1,41 @@
+---
+description: The <ClientOnly> component renders its slot only in client-side.
+---
+# `<ClientOnly>`
+
+The `<ClientOnly>` component renders its slot only in client-side. To import a component only on the client, register the component in a client-side only plugin.
+
+## Props
+
+- **placeholderTag** | **fallbackTag**: specify a tag to be rendered server-side.
+- **placeholder** | **fallback**: specify a content to be rendered server-side.
+
+```vue
+<template>
+  <div>
+    <Sidebar />
+    <ClientOnly fallback-tag="span" fallback="Loading comments...">
+      <Comment />
+    </ClientOnly>
+  </div>
+</template>
+```
+
+## Slots
+
+- **#fallback**: specify a content to be displayed server-side.
+
+```vue
+<template>
+  <div>
+    <Sidebar />
+    <ClientOnly>
+      <!-- ... -->
+      <template #fallback>
+        <!-- this will be rendered on server side -->
+        <p>Loading comments...</p>
+      </template>
+    </ClientOnly>
+  </div>
+</template>
+```