Fixed #30711 -- Doc'd django.contrib.postgres.fields.hstore.KeyTransform().

docs/ref/contrib/postgres/fields.txt

@@ -319,6 +319,22 @@ transform do not change. For example::
     valid for a given field. This can be done using the
     :class:`~django.contrib.postgres.validators.KeysValidator`.
 
+KeyTransform() expression
+-------------------------
+
+.. class:: hstore.KeyTransform(key_name, *args, **kwargs)
+
+Returns the value of the given ``key_name``. This allows you to annotate a key
+value. For example::
+
+    >>> from django.contrib.postgres.fields.hstore import KeyTransform
+    >>> Dog.objects.create(name="Rufus", data={"breed": "labrador"})
+    >>> Dog.objects.create(name="Meg", data={"breed": "collie", "owner": "Bob"})
+
+    >>> rufus = Dog.objects.annotate(breed=KeyTransform("breed", "data"))[0]
+    >>> rufus.breed
+    'labrador'
+
 Querying ``HStoreField``
 ------------------------