Add `short_help` option to `clap.main_factory()` function

* The `clap.main_factory()` function now accepts a `short_help` keyword option.
* This option accepts values of type `bool | str` and defaults to `False` (uses first line of `__doc__` if `True` is provided).
* If this option is provided, the final CLI will require you use `--help` for the full help documentation (`-h` will show only a subset of the documentation that `--help` will show).