bpo-8264: Document hasattr and getattr behavior for private attributes (GH-23513)

Clarify ``getattr`` and ``setattr`` requirements for accessing name-mangled attributes

Co-Authored-By: Catalin Iacob <iacobcatalin@gmail.com>
diff --git a/Doc/reference/expressions.rst b/Doc/reference/expressions.rst
index 8ac6264..c8c9b46 100644
--- a/Doc/reference/expressions.rst
+++ b/Doc/reference/expressions.rst
@@ -77,6 +77,8 @@
 When a name is not bound, an attempt to evaluate it raises a :exc:`NameError`
 exception.
 
+.. _private-name-mangling:
+
 .. index::
    pair: name; mangling
    pair: private; names