Document client address limitation

Addresses #871

Author: ST-DDT

docs/en/client/configuration.md

@@ -102,6 +102,8 @@ If you don't define an address it will be guessed:
 > **Note:** The number of slashes is important! Also make sure that you don't try to connect to a normal
 > web/REST/non-grpc server (port).
 
+> **Note:** Java does not support addresses with underscores (`_`): `dns://user_service`. Please make sure you use hyphens (`-`) instead.
+
 The `SSL`/`TLS` and other security relevant configuration is explained on the [Client Security](security.md) page.
 
 ## Configuration via Beans