PEP 726: Correct and expand specification (#3320)

Co-authored-by: Oscar Benjamin <oscar.j.benjamin@gmail.com>
Co-authored-by: Jelle Zijlstra <jelle.zijlstra@gmail.com>
Co-authored-by: Adam Turner <9087854+aa-turner@users.noreply.github.com>

pep-0726.rst

@@ -134,9 +134,47 @@ module ``__dict__``.  If present, the appropriate function is called to
 customize setting the attribute or its deletion, else the normal
 mechanism (storing/deleting the value in the module dictionary) will work.
 
-Defining ``__setattr__`` or ``__delattr__`` only affect lookups made
-using the attribute access syntax---directly accessing the module
-globals is unaffected, e.g. ``sys.modules[__name__].some_global = 'spam'``.
+Defining module ``__setattr__`` or ``__delattr__`` only affects lookups made
+using the attribute access syntax---directly accessing the module globals
+(whether by ``globals()`` within the module, or via a reference to the module's
+globals dictionary) is unaffected.  For example:
+
+.. code:: pycon
+
+   >>> import mod
+   >>> mod.__dict__['foo'] = 'spam'  # bypasses __setattr__, defined in mod.py
+
+or
+
+.. code:: python
+
+   # mod.py
+
+   def __setattr__(name, value):
+      ...
+
+   foo = 'spam'  # bypasses __setattr__
+   globals()['bar'] = 'spam'  # here too
+
+   def f():
+       global x
+       x = 123
+
+   f()  # and here
+
+To use a module global and trigger ``__setattr__`` (or ``__delattr__``),
+one can access it via ``sys.modules[__name__]`` within the module's code:
+
+.. code:: python
+
+   # mod.py
+
+   sys.modules[__name__].foo = 'spam'  # bypasses __setattr__
+
+   def __setattr__(name, value):
+       ...
+
+   sys.modules[__name__].bar = 'spam'  # triggers __setattr__
 
 
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