[1.8.x] Fixed #25531 -- Documented that admin_order_field supports lookups.

Backport of 2c727101 from master

docs/ref/contrib/admin/index.txt

@@ -677,6 +677,22 @@ subclass::
 
           colored_first_name.admin_order_field = '-first_name'
 
+      ``admin_order_field`` supports query lookups to sort by values on related
+      models. This example includes an "author first name" column in the list
+      display and allows sorting it by first name::
+
+          class Blog(models.Model):
+              title = models.CharField(max_length=255)
+              author = models.ForeignKey(Person, on_delete=models.CASCADE)
+
+          class BlogAdmin(admin.ModelAdmin):
+              list_display = ('title', 'author', 'author_first_name')
+
+              def author_first_name(self, obj):
+                  return obj.author.first_name
+
+              author_first_name.admin_order_field = 'author__first_name'
+
     * Elements of ``list_display`` can also be properties. Please note however,
       that due to the way properties work in Python, setting
       ``short_description`` on a property is only possible when using the