Add type signature for methods with no overloads

Author: egregius313

csharp/ql/lib/ext/System.model.yml

@@ -6,9 +6,9 @@ extensions:
       - ["System", "Console", False, "Read", "", "", "ReturnValue", "local", "manual"]
       - ["System", "Console", False, "ReadKey", "", "", "ReturnValue", "local", "manual"]
       - ["System", "Console", False, "ReadLine", "", "", "ReturnValue", "local", "manual"]
-      - ["System", "Environment", False, "ExpandEnvironmentVariables", "", "", "ReturnValue", "environment", "manual"]
-      - ["System", "Environment", False, "GetCommandLineArgs", "", "", "ReturnValue", "commandargs", "manual"]
-      - ["System", "Environment", False, "get_CommandLine", "", "", "ReturnValue", "commandargs", "manual"]
+      - ["System", "Environment", False, "ExpandEnvironmentVariables", "(System.String)", "", "ReturnValue", "environment", "manual"]
+      - ["System", "Environment", False, "GetCommandLineArgs", "()", "", "ReturnValue", "commandargs", "manual"]
+      - ["System", "Environment", False, "get_CommandLine", "()", "", "ReturnValue", "commandargs", "manual"]
       - ["System", "Environment", False, "GetEnvironmentVariable", "", "", "ReturnValue", "environment", "manual"]
       - ["System", "Environment", False, "GetEnvironmentVariables", "", "", "ReturnValue", "environment", "manual"]
   - addsTo: