Added a single line to explain what the server_name is used for

synapse/config/server.py

@@ -133,6 +133,7 @@ class ServerConfig(Config):
         # The domain name of the server, with optional explicit port.
         # This is used by remote servers to connect to this server,
         # e.g. matrix.org, localhost:8080, etc.
+        # This is also the last part of your UserID.
         server_name: "%(server_name)s"
 
         # When running as a daemon, the file to store the pid in