Alert Pattern and example: Reference live region guidance for explanation of changes that trigger live events

[mcking65]

The alert ppattern does not explain what types of DOM changes will trigger an alert to be spoken.
This topic will be addressed in the live region guidance section being drafted for issue #78 in PR #1027.

After #1027 is merged, add a note that references the explanation of the types of changes that trigger alerts to the roles, states, and properties section of the alert pattern.

Also add an explanation to the accessibility features of the example and a reference to the same guidance.