The parameter help in the method add_parser of _SubParsersAction (that we obtain with ArgumentParser.add_subparsers()) Is not documented.

In the documentation we can find:
The add_subparsers() method is normally called with no arguments and returns a special action object. This object has a single method, add_parser(), which takes a command name and any ArgumentParser constructor arguments, and returns an ArgumentParser object that can be modified as usual.

I found the parameter by accident and then I checked in the source code, it's unfortunate because the help parameter give really nice output when having sub commands.

the parameter 'aliases' is also not documented.