[Email Security] Add note around auto-sync (#19909)

Author: Maddy-Cloudflare

src/content/docs/cloudflare-one/email-security/directories/index.mdx

@@ -15,4 +15,8 @@ To add a directory:
 4. Select **Add a directory** > **Manage integrations**.
 5. On the SaaS integrations page, select **Add integration**.
 
-To sync a directory, select the three dots, then select **Sync now**.
+To sync a directory, select the three dots, then select **Sync now**.
+
+:::note
+The **Auto sync** option is on by default. It is recommended to keep this option on at all times to ensure directories are always synchronized.
+:::