Module-level variable "docstrings" not picked up

@mhvk - I see you've implemented an option to add module-level variables to automodapi in #24 .
This is something I've wanted for Gammapy for a long time, thank you!

One question I have is how one should document those variables.
In the test case you add in #24 you have
```
PARROT_STATE = 'dead'
"""The global state of the parrot."""
```
and I would have expected `"""The global state of the parrot."""` to show up in the HTML docs.
But instead this is shown: `str(object=’‘) -> str`.
So apparently `PARROT_STATE.__doc__` is used directly.

What should we do here?
Change Sphinx to find and use the string following the variable in the output?
Or just write "variable docstrings" like this?
```
PARROT_STATE = 'dead'
PARROT_STATE.__doc__ = """The global state of the parrot."""
```

Whatever decision is made, I think this should be mentioned in the docs, and maybe even the test should be changed to make sure `PARROT_STATE.__doc__` is picked up correctly by Sphinx.

cc @astrofrog - Maybe you have an opinion here as well?

Start by reviewing the module-level-variable implementation and test case introduced in #24, then compare the generated HTML with the variable's following string and its __doc__ value. Decide which documentation convention Sphinx should support, update the documentation to describe it, and change or add the test so the chosen variable documentation appears correctly in the HTML output.

Written by the indexing model from the issue text.