Functions that are decorated with `@lru_cache` or other `functools`
decorators may not even be detected as a function. This results in the
documentation not having the `()` or even trying to render the function
signature.
This change updates the `sphinx.util.inspect` code to unwrap
`__wrapped__` functions before determining if they can be documented.
`@lru_cache` and its associated test is an example of a decorated
function that is incorrectly identified as an attribute rather than a
module level function and when rendering the signature (upon changing
`isattributedescriptor`) the decorated function is still incorrectly
identified as a C function.
This change also renames the newly introduced `unwrap` as `unwrap_all`
because it is different than the prexisting Python supplied
`inspect.unwrap`.
See `update_wrapper` "Changed in version 3.4" for more background:
https://docs.python.org/3/library/functools.html#functools.update_wrapper
