docs(Group): document batch operations and tri-state slot props

Author: johnleider

apps/docs/src/pages/components/providers/group.md

@@ -35,6 +35,47 @@ Button items in a group with shared multi-selection state — toggling any item
 
 :::
 
+## Features
+
+### Batch Operations
+
+The default slot exposes `selectAll`, `unselectAll`, and `toggleAll` methods for operating on all items at once:
+
+```vue
+<template>
+  <Group.Root v-model="selected">
+    <template #default="{ selectAll, unselectAll, toggleAll, isAllSelected, isMixed }">
+      <button @click="toggleAll">
+        {{ isAllSelected ? 'Deselect All' : 'Select All' }}
+      </button>
+
+      <Group.Item v-for="item in items" :key="item" :value="item">
+        {{ item }}
+      </Group.Item>
+    </template>
+  </Group.Root>
+</template>
+```
+
+### Tri-State
+
+The slot props `isAllSelected`, `isNoneSelected`, and `isMixed` reflect the aggregate selection state — useful for driving an "indeterminate" checkbox:
+
+```vue
+<template>
+  <Group.Root v-model="selected">
+    <template #default="{ isAllSelected, isMixed, toggleAll }">
+      <input
+        type="checkbox"
+        :checked="isAllSelected"
+        :indeterminate="isMixed"
+        @change="toggleAll"
+      />
+    </template>
+  </Group.Root>
+</template>
+```
+
 ## Anatomy
 
 ```vue Anatomy playground