Refs #30278 -- Doc'd behavior of del on an unaccessed cached_property.

Thanks to Curtis Maloney for the description of the problem.

Author: schinckel

docs/ref/utils.txt

@@ -473,6 +473,10 @@ https://web.archive.org/web/20110718035220/http://diveintomark.org/archives/2004
  # set a value manually, that will persist on the instance until cleared
  person.friends = ["Huckleberry Finn", "Tom Sawyer"]
 
+ Because of the way the `descriptor protocol
+ <descriptor-invocation>`_ works, using ``del`` (or ``delattr``) on a
+ ``cached_property`` that hasn't been accessed raises ``AttributeError``.
+
  As well as offering potential performance advantages, ``@cached_property``
  can ensure that an attribute's value does not change unexpectedly over the
  life of an instance. This could occur with a method whose computation is