docs(material/datepicker): a11y, require confirmation buttons

Datepicker accessiblity docs require proving confirmation buttons.

Author: zarend

src/material/datepicker/datepicker.md

@@ -566,6 +566,8 @@ The `MatDatepicker` pop-up uses the `role="dialog"` interaction pattern. This di
 multiple controls, the most prominent being the calendar itself. This calendar implements the
 `role="grid"` interaction pattern.
 
+The `matDatepickerCanel` and `matDatepickerApply` provide confirmation of the selected date when closing the pop-up. Always enable the confirmation action buttons (see  [_Confirmation action buttons_](#confirmation-action-buttons)).
+
 The `MatDatepickerInput` and `MatDatepickerToggle` directives both apply the `aria-haspopup`
 attribute to the native input and button elements, respectively.