Fixed #21127 -- Started deprecation toward requiring on_delete for ForeignKey/OneToOneField

docs/ref/contrib/admin/admindocs.txt

@@ -74,8 +74,12 @@ A model with useful documentation might look like this::
 
         """
         slug = models.SlugField(help_text="A short label, generally used in URLs.")
-        author = models.ForeignKey(User)
-        blog = models.ForeignKey(Blog)
+        author = models.ForeignKey(
+            User,
+            models.SET_NULL,
+            blank=True, null=True,
+        )
+        blog = models.ForeignKey(Blog, models.CASCADE)
         ...
 
         def publish(self):

docs/ref/contrib/admin/index.txt

@@ -1957,7 +1957,7 @@ information.
             name = models.CharField(max_length=100)
 
          class Book(models.Model):
-            author = models.ForeignKey(Author)
+            author = models.ForeignKey(Author, on_delete=models.CASCADE)
             title = models.CharField(max_length=100)
 
     You can edit the books authored by an author on the author page. You add
@@ -2174,8 +2174,8 @@ Take this model for instance::
     from django.db import models
 
     class Friendship(models.Model):
-        to_person = models.ForeignKey(Person, related_name="friends")
-        from_person = models.ForeignKey(Person, related_name="from_friends")
+        to_person = models.ForeignKey(Person, on_delete=models.CASCADE, related_name="friends")
+        from_person = models.ForeignKey(Person, on_delete=models.CASCADE, related_name="from_friends")
 
 If you wanted to display an inline on the ``Person`` admin add/change pages
 you need to explicitly define the foreign key since it is unable to do so
@@ -2281,8 +2281,8 @@ models::
         members = models.ManyToManyField(Person, through='Membership')
 
     class Membership(models.Model):
-        person = models.ForeignKey(Person)
-        group = models.ForeignKey(Group)
+        person = models.ForeignKey(Person, on_delete=models.CASCADE)
+        group = models.ForeignKey(Group, on_delete=models.CASCADE)
         date_joined = models.DateField()
         invite_reason = models.CharField(max_length=64)
 
@@ -2326,7 +2326,7 @@ you have the following models::
 
     class Image(models.Model):
         image = models.ImageField(upload_to="images")
-        content_type = models.ForeignKey(ContentType)
+        content_type = models.ForeignKey(ContentType, on_delete=models.CASCADE)
         object_id = models.PositiveIntegerField()
         content_object = GenericForeignKey("content_type", "object_id")