Change docstring params to be usable in sphinx
Fix docstring usage example of ArgumentGroup
Update docstring of SubparserGroup method
Add docstring for SubparserGroup add_to_parser
Allow to set usage, description and epilog on a SubParser
Allow to call self when subcommand is parsed
Add SubparsersMixin has_subparsers method

With this change it will be possible to call a Subparser as an argparse func
Allow to set description and usage on a Parser class
Add usage example for Parser class in docstring
Update Subparser usage example in docstring

Also show an example for adding subparser declerativly.