add smtp helper to docs

Author: msukkari

docs/self-hosting/configuration/transactional-emails.mdx

@@ -11,5 +11,5 @@ To enable transactional emails in your deployment, set the following environment
 
 | Variable | Description |
 | :------- | :---------- |
-| `SMTP_CONNECTION_URL` | SMTP server connection. |
+| `SMTP_CONNECTION_URL` | SMTP server connection (`smtp://[user[:password]@]host[:port]`)|
 | `EMAIL_FROM_ADDRESS` | The sender's email address |