Shorten docstring for warn_about_none_ast

src/_pytest/assertion/rewrite.py

@@ -878,16 +878,10 @@ class AssertionRewriter(ast.NodeVisitor):
         return self.statements
 
     def warn_about_none_ast(self, node, module_path, lineno):
-        """Returns an ast warning if node is None with the following statement:
-        if node is None:
-            from _pytest.warning_types import PytestWarning
-            import warnings
-            warnings.warn_explicit(
-                PytestWarning('assertion the value None, Please use "assert is None"'),
-                category=None,
-                filename=str,
-                lineno=node.lineno,
-            )
+        """
+        Returns an ast issuing a warning if the value of node is `None`
+        This is used to warn the user when asserting a function that asserts internally.
+        See issue #3191 for more details
         """
 
         # using parse because it's different between py2 py3