Fix #8545: autodoc: a __slots__ attribute is not documented even having docstring

To avoid filtering __slots__ attributes having docstring at
filter_members(), this passes docstring captured at get_class_members()
to the filter_members() via ObjectMember.

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

@@ -1,8 +1,12 @@
 class Foo:
+    """docstring"""
+
     __slots__ = ['attr']
 
 
 class Bar:
+    """docstring"""
+
     __slots__ = {'attr1': 'docstring of attr1',
                  'attr2': 'docstring of attr2',
                  'attr3': None}
@@ -12,4 +16,6 @@ class Bar:
 
 
 class Baz:
+    """docstring"""
+
     __slots__ = 'attr'