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

For the datepicker accessiblity documentation, add instructions to always
provide confirmation buttons. This prevents an accessibility issue where
selecting a date on the calendar gives no audio feedback of the selected date
(#23568).

This commit only affects the docs and does not change source code.

Addresses #23568

Author: zarend

src/material/datepicker/datepicker.md

@@ -566,6 +566,10 @@ 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 `matDatepickerCancel` and `matDatepickerApply` provide both visual and audio 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.