Clarify environment variable usage when running in Docker (#6181)

docker/start.py

@@ -217,8 +217,9 @@ def main(args, environ):
         # backwards-compatibility generate-a-config-on-the-fly mode
         if "SYNAPSE_CONFIG_PATH" in environ:
             error(
-                "SYNAPSE_SERVER_NAME and SYNAPSE_CONFIG_PATH are mutually exclusive "
-                "except in `generate` or `migrate_config` mode."
+                "SYNAPSE_SERVER_NAME can only be combined with SYNAPSE_CONFIG_PATH "
+                "in `generate` or `migrate_config` mode. To start synapse using a "
+                "config file, unset the SYNAPSE_SERVER_NAME environment variable."
             )
 
         config_path = "/compiled/homeserver.yaml"