Update env file warning message to be more specific

lightrag/api/utils_api.py

@@ -25,7 +25,7 @@ def check_env_file():
     """
     if not os.path.exists(".env"):
         warning_msg = (
-            "Warning: .env file not found. Some features may not work properly."
+            "Warning: Startup directory must contain .env file for multi-instance support."
         )
         ASCIIColors.yellow(warning_msg)