minor update to python generator usage (#762)

Author: wing328

modules/openapi-generator/src/main/java/org/openapitools/codegen/languages/PythonClientCodegen.java

@@ -165,7 +165,7 @@ public PythonClientCodegen() {
         supportedLibraries.put("urllib3", "urllib3-based client");
         supportedLibraries.put("asyncio", "Asyncio-based client (python 3.5+)");
         supportedLibraries.put("tornado", "tornado-based client");
-        CliOption libraryOption = new CliOption(CodegenConstants.LIBRARY, "library template (sub-template) to use");
+        CliOption libraryOption = new CliOption(CodegenConstants.LIBRARY, "library template (sub-template) to use: asyncio, tornado, urllib3");
         libraryOption.setDefault(DEFAULT_LIBRARY);
         cliOptions.add(libraryOption);
         setLibrary(DEFAULT_LIBRARY);