missing-*-docstring can look for __doc__ assignments.

Close #3301

ChangeLog

@@ -25,6 +25,10 @@ Release date: TBA
 
   Close #560
 
+* ``missing-*-docstring`` can look for ``__doc__`` assignments.
+
+   Close #3301
+
 * ``safe_infer`` can infer a value as long as all the paths share the same type.
 
   Close #2503

pylint/checkers/base.py

@@ -2097,6 +2097,9 @@ def _check_docstring(
     ):
         """check the node has a non empty docstring"""
         docstring = node.doc
+        if docstring is None:
+            docstring = _infer_dunder_doc_attribute(node)
+
         if docstring is None:
             if not report_missing:
                 return
@@ -2167,6 +2170,21 @@ def _is_one_arg_pos_call(call):
     return isinstance(call, astroid.Call) and len(call.args) == 1 and not call.keywords
 
 
+def _infer_dunder_doc_attribute(node):
+    # Try to see if we have a `__doc__` attribute.
+    try:
+        docstring = node["__doc__"]
+    except KeyError:
+        return None
+
+    docstring = utils.safe_infer(docstring)
+    if not docstring:
+        return None
+    if not isinstance(docstring, astroid.Const):
+        return None
+    return docstring.value
+
+
 class ComparisonChecker(_BasicChecker):
     """Checks for comparisons
 

tests/functional/m/missing_docstring.py

@@ -52,3 +52,7 @@ def test(self, value):
     @test.deleter
     def test(self):
         pass
+
+
+class DocumentedViaDunderDoc(object):
+    __doc__ = "This one"