email: populate description field in TB docs

At least one user was confused and put submission.torproject.org, and
used torproject.org as the server name.

service/email.md

@@ -10,6 +10,7 @@ records are setup.
 In general, you can configure your email client with the following
 SMTP settings:
 
+ * Description: `torproject.org`
  * Server name: `submission.torproject.org`
  * Port: `465`
  * Connection security: `TLS`