Alert: Document dismissible alert persistence

src/_components/alert/index.md

@@ -67,6 +67,7 @@ Used when an action link is needed in place of a standard link.
 
 * Any alert variation can be dismissible, including slim alerts. This example shows an informational alert that can be dismissed.
 * Allow a user to dismiss an alert wherever appropriate.
+* By default, dismissible alerts do not persist their dismissed state across sessions. If you need an alert to remain dismissed after a user closes their browser, your application must implement state persistence.
 {% include a11y/dismissable-alerts.md component="alert" %}
 
 ### Slim alert
@@ -100,6 +101,7 @@ Any style of alert box can be modified to be a Slim alert. The iconography for S
 
 * Any alert variation can be dismissible. This example shows an informational alert that can be dismissed.
 * Allow a user to dismiss an alert wherever appropriate.
+* By default, dismissible alerts do not persist their dismissed state across sessions. If you need an alert to remain dismissed after a user closes the app, your application must implement state persistence.
 
 ### Expandable