add (#10359)

Author: rickyk586

src/components/dialog/dialog.js

@@ -549,6 +549,7 @@ function MdDialogDirective($$rAF, $mdTheming, $mdDialog) {
  *      in parallel the close animations.
  *   - `fullscreen` `{boolean=}`: An option to toggle whether the dialog should show in fullscreen
  *      or not. Defaults to `false`.
+ *   - `multiple` `{boolean=}`: An option to allow this dialog to display over one that's currently open.
  * @returns {promise} A promise that can be resolved with `$mdDialog.hide()` or
  * rejected with `$mdDialog.cancel()`.
  */