Fix #9866: autodoc: doccoment for the imported class was ignored

Autodoc tried to scan doccomment on the module where the class defined.
But it failed to get it if the class is imported from other module.

This analyzes the target module to obtain the doccomment.

tests/test_ext_autodoc_autoclass.py

@@ -407,6 +407,18 @@ def test_class_alias_having_doccomment(app):
     ]
 
 
+def test_class_alias_for_imported_object_having_doccomment(app):
+    actual = do_autodoc(app, 'class', 'target.classes.IntAlias')
+    assert list(actual) == [
+        '',
+        '.. py:attribute:: IntAlias',
+        '   :module: target.classes',
+        '',
+        '   docstring',
+        '',
+    ]
+
+
 @pytest.mark.sphinx('html', testroot='ext-autodoc')
 def test_coroutine(app):
     options = {"members": None}