Add documentation of SMTP_DOMAIN (#1738)

Without setting it, exim will reject the mail with a message like:
rejected EHLO from [10.20.0.1]: syntactically invalid argument(s): {}

.env.production.sample

@@ -42,6 +42,7 @@ SMTP_PORT=587
 SMTP_LOGIN=
 SMTP_PASSWORD=
 SMTP_FROM_ADDRESS=notifications@example.com
+#SMTP_DOMAIN= # defaults to LOCAL_DOMAIN
 #SMTP_DELIVERY_METHOD=smtp # delivery method can also be sendmail
 #SMTP_AUTH_METHOD=plain
 #SMTP_OPENSSL_VERIFY_MODE=peer