Fixed #30759 -- Made cache.delete() return whether it succeeded.

Thanks Simon Charette for the review.

docs/releases/3.1.txt

@@ -118,6 +118,9 @@ Cache
   field names in the ``no-cache`` directive for the ``Cache-Control`` header,
   according to :rfc:`7234#section-5.2.2.2`.
 
+* :meth:`~django.core.caches.cache.delete` now returns ``True`` if the key was
+  successfully deleted, ``False`` otherwise.
+
 CSRF
 ~~~~