add one more

Author: andrewseguin

src/lib/list/list.ts

@@ -89,6 +89,7 @@ export class MdListIconCssMatStyler {}
 
 /**
  * Directive whose purpose is to add the mat- CSS styling to this selector.
+ * @docs-private
  */
 @Directive({
   selector: '[md-subheader], [mat-subheader]',