add documentation for availability and differences for `autodoc-process-docstring` (and some friends)

Start with the autodoc-process-docstring and autodoc-process-signature events in the Sphinx extension, and verify whether AutoAPI emits them and how their what, obj, and options values differ. Update the documentation to list event availability and supported what values for autodoc and AutoAPI. Done when these behaviors and differences are clear to users.

Hi,

I am in need for some module docstring post-processing, and it took me rather long to figure out that:

1. autoapi also allows (some?) autodoc-* events to be captured
2. There are some differences in the values for what

Especially it took me quite a while to figure out there are some subtle differences:

• autodoc: what can be : "module", "class", "exception", "function", "method", "attribute"
• autoapi: what can be : "package", "class", "exception", "function", "method", "attribute", "data"

Im not quite sure if t

I think that would be good to add to the documentation if this is indeed the intent, rather than a side effect of something else in my setup
It appears that the autodoc-process-signature event is also available, but I have not done any further validation on that, but as it appears to be used internally by autoapi, it is likely to work as well.

extensions = [
    "autoapi.extension",
    "sphinx.ext.napoleon",
]

# ...
mpy_lib_modules = {"foo":"bar"}

def process_docstring(
    app: Sphinx,
    what: str,  # "module", "class", "exception", "function", "method", "attribute" ( "package", 'data' with autoapi)
    name: str,
    obj,  # Always None with autoapi
    options,  # Always None with autoapi
    lines: List[str],
):
    if what in {"package", "module"} and name in mpy_lib_modules:
            lines.extend(
                (
                    "",
                    f".. seealso:: This is a `{mpy_lib_modules[name]}` module from the `micropython-lib` repository.",
                )
            )

def process_signature(app, what, name, obj, options, signature, return_annotation):
    pass

def setup(sphinx: Sphinx):
    sphinx.connect("autodoc-process-docstring", process_docstring) # also fires with autoapi :)
    sphinx.connect("autodoc-process-signature", process_signature)  # also fires with autoapi :)