Allow to place content inside home `<section>`

[mondeja]

Currently there is no way to place content inside the same top level section created by sphinx-argparse-cli. If you try to pass content to the directive like:

.. sphinx_argparse_cli::
   :module: my_package.__main__
   :func: build_parser

   Some content

...the next error is raised:

Warning, treated as error:
/my/own/file.rst:1:Error in "sphinx_argparse_cli" directive:
no content permitted.

So it seems legit to allow adding content to the directive that would be placed inside the home section.

---

More context

This is specially useful when you want to add another section at the same section level of a subsection created by sphinx-argparse-cli, for example <prog> optional arguments.

Currently this is not possible at all, no matter which heading syntax you use, the next section after the directive will be placed at the same level of the home section.

[gaborbernat]

Feel free to add a PR to achieve this 👍