Allow to mark function from the docstring when using autodoc

[Jackenmen]

Similarly to how :meta private:, :meta public:, or :meta hide-value: in Sphinx work (see the docs here), I would want to be able to mark function as a decorator/abstractmethod/etc using something like this:

def function():
    """
    Description of the method

    :trio decorator:
    :trio classmethod:
    """

It should probably be possible to base its implementation on Sphinx's own implementation used for the :meta: fields.

[Jackenmen]

Here's what I came up with:
Jackenmen@d75f07b

It does seem to work but I have not done very thorough testing since I just wanted to first check how difficult it would be to implement this.

I can work on this more if this feature request gets accepted.