docs: Enhanced description of "direct reply" workflow.

The "Direct Reply" section of the "Notifications" page within the
N Preview documentation now includes a code snippet describing how
to show an acknowledgment of a user's response within a notification.
Also, the "chat history" example now has a clearer, more complete
explanation.

Bug: 27665098
Change-Id: I2fd098ab30a00f4fa2a84f3b150cc1c7f7f53750