Email body only supports plain text messages

doc/tools/email.md

@@ -22,7 +22,9 @@ at their primary email address.
    ![admin users](email1.png)
 
 1. Compose an email and choose where it will be sent (all users or users of a
-   chosen group or project):
+   chosen group or project). The email body only supports plain text messages.
+   HTML, Markdown, and other rich text formats are not supported, and will be
+   sent as plain text to users.
 
    ![compose an email](email2.png)