[1.5.X] Fixed #20183 - Clarified docs assumption that certain objects exist in database.

Thanks Tomasz Jaskowski for the patch.

Backport of 41bea033b7 from master

docs/topics/db/queries.txt

@@ -100,7 +100,8 @@ Saving ``ForeignKey`` and ``ManyToManyField`` fields
 Updating a :class:`~django.db.models.ForeignKey` field works exactly the same
 way as saving a normal field -- simply assign an object of the right type to
 the field in question. This example updates the ``blog`` attribute of an
-``Entry`` instance ``entry``::
+``Entry`` instance ``entry``, assuming appropriate instances of ``Entry`` and
+``Blog`` are already saved to the database (so we can retrieve them below)::
 
     >>> from blog.models import Entry
     >>> entry = Entry.objects.get(pk=1)