Fixed #28653 -- Added missing ForeignKey.on_delete argument in docs.

docs/ref/models/querysets.txt

@@ -1064,7 +1064,7 @@ fields. Suppose we have an additional model to the example above::
 
     class Restaurant(models.Model):
         pizzas = models.ManyToManyField(Pizza, related_name='restaurants')
-        best_pizza = models.ForeignKey(Pizza, related_name='championed_by')
+        best_pizza = models.ForeignKey(Pizza, related_name='championed_by', on_delete=models.CASCADE)
 
 The following are all legal:
 

docs/topics/conditional-view-processing.txt

@@ -80,7 +80,7 @@ Suppose you have this pair of models, representing a simple blog system::
         ...
 
     class Entry(models.Model):
-        blog = models.ForeignKey(Blog)
+        blog = models.ForeignKey(Blog, on_delete=models.CASCADE)
         published = models.DateTimeField(default=datetime.datetime.now)
         ...
 

docs/topics/db/aggregation.txt

@@ -34,7 +34,7 @@ used to track the inventory for a series of online bookstores:
         price = models.DecimalField(max_digits=10, decimal_places=2)
         rating = models.FloatField()
         authors = models.ManyToManyField(Author)
-        publisher = models.ForeignKey(Publisher)
+        publisher = models.ForeignKey(Publisher, on_delete=models.CASCADE)
         pubdate = models.DateField()
 
     class Store(models.Model):

docs/topics/db/queries.txt

@@ -34,7 +34,7 @@ models, which comprise a Weblog application:
             return self.name
 
     class Entry(models.Model):
-        blog = models.ForeignKey(Blog)
+        blog = models.ForeignKey(Blog, on_delete=models.CASCADE)
         headline = models.CharField(max_length=255)
         body_text = models.TextField()
         pub_date = models.DateField()