docs: updated multiselect docs with select all feature details and us…

…age (#17758)

* docs: updated multiselect docs with select all feature usage

* Update packages/react/src/components/MultiSelect/MultiSelect.mdx

Co-authored-by: kennylam <909118+kennylam@users.noreply.github.com>

* Update packages/react/src/components/MultiSelect/MultiSelect.mdx

Co-authored-by: Alison Joseph <alison.joseph@us.ibm.com>

* Update packages/react/src/components/MultiSelect/MultiSelect.mdx

Co-authored-by: Alison Joseph <alison.joseph@us.ibm.com>

* docs: added select all to navigation

---------

Co-authored-by: kennylam <909118+kennylam@users.noreply.github.com>
Co-authored-by: Alison Joseph <alison.joseph@us.ibm.com>

packages/react/src/components/MultiSelect/MultiSelect.mdx

@@ -19,6 +19,7 @@ import * as MultiSelect from './MultiSelect.stories';
 
 - [Overview](#overview)
   - [With initial selected items](#with-initial-selected-items)
+  - [Select all](#select-all)
   - [Filtering](#filtering)
   - [With layer](#with-layer)
 - [Component API](#component-api)
@@ -42,6 +43,29 @@ By passing items into the `initialSelectedItems` prop, they can be pre-selected.
 
 <Canvas of={MultiSelect.WithInitialSelectedItems} />
 
+### Select all
+
+The MultiSelect component includes a "Select All" feature, allowing users to
+easily select or deselect all options with a single action.
+
+To implement the "Select All" feature in your MultiSelect component, add a new
+option to your options array with these properties:
+
+- `id`: A unique identifier (e.g., 'select-all')
+- `text`: The label for the option
+- `isSelectAll`: Set to `true`
+
+```
+const options = [
+  {
+    id: 'select-all',
+    text: 'All roles',
+    isSelectAll: true,
+  },
+  // ... other options
+];
+```
+
 ### Filtering
 
 The `FilterableMultiSelect` component allows for filtering the list of