docs(CCollapse): update API documentation

Author: mrholek

packages/docs/content/4.0/api/CCollapse.api.mdx

@@ -8,6 +8,7 @@ import CCollapse from '@coreui/react/src/components/collapse/CCollapse'
 | Property | Description | Type | Default |
 | --- | --- | --- | --- |
 | **className** | A string of all className you want applied to the base component. | `string` | - |
+| **horizontal** | Set horizontal collapsing to transition the width instead of height. | `boolean` | - |
 | **onHide** | Callback fired when the component requests to be hidden. | `() => void` | - |
 | **onShow** | Callback fired when the component requests to be shown. | `() => void` | - |
 | **visible** | Toggle the visibility of component. | `boolean` | - |