Fix: autodoc: __wrapped__ functions are not documented correctly

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

tests/roots/test-ext-autodoc/target/wrappedfunction.py

@@ -0,0 +1,8 @@
+# for py32 or above
+from functools import lru_cache
+
+
+@lru_cache(maxsize=None)
+def slow_function(message, timeout):
+    """This function is slow."""
+    print(message)

tests/test_autodoc.py

@@ -1369,6 +1369,19 @@ def test_partialmethod(app):
     assert list(actual) == expected
 
 
+@pytest.mark.sphinx('html', testroot='ext-autodoc')
+def test_wrappedfunction(app):
+    actual = do_autodoc(app, 'function', 'target.wrappedfunction.slow_function')
+    assert list(actual) == [
+        '',
+        '.. py:function:: slow_function(message, timeout)',
+        '   :module: target.wrappedfunction',
+        '',
+        '   This function is slow.',
+        '   ',
+    ]
+
+
 @pytest.mark.sphinx('html', testroot='ext-autodoc')
 def test_partialmethod_undoc_members(app):
     expected = [