docs(cdk): add cdk-drag-handle to Styling section (#15774)

Author: jerome-nelson

src/cdk/drag-drop/drag-drop.md

@@ -73,6 +73,7 @@ by the directives:
 | `.cdk-drop-list`    | Corresponds to the `cdkDropList` container.                              |
 | `.cdk-drag`         | Corresponds to a `cdkDrag` instance.                                     |
 | `.cdk-drag-disabled`| Class that is added to a disabled `cdkDrag`.                             |
+| `.cdk-drag-handle`  | Class that is added to the host element of the cdkDragHandle directive.  | 
 | `.cdk-drag-preview` | This is the element that will be rendered next to the user's cursor as they're dragging an item in a sortable list. By default the element looks exactly like the element that is being dragged. |
 | `.cdk-drag-placeholder` | This is element that will be shown instead of the real element as it's being dragged inside a `cdkDropList`. By default this will look exactly like the element that is being sorted. |
 | `.cdk-drop-list-dragging` | A class that is added to `cdkDropList` while the user is dragging an item.  |