[Email Security] Add text add-ons warning

Author: Maddy-Cloudflare

src/content/docs/cloudflare-one/email-security/detection-settings/configure-text-add-ons.mdx

@@ -7,6 +7,10 @@ sidebar:
 
 You can create custom labels to be used as the subject or body prefix for emails with specific dispositions.
 
+:::caution
+Enabling text add-ons will add a banner to the subject line, causing the subject line to be reduced.
+:::
+
 ## Subject prefix
 
 To configure a subject prefix: