[1.6.x] Added warning about get_inline_instances() permission checking; refs #23754.

Backport of e0d1f2684a from master

docs/ref/contrib/admin/index.txt

@@ -1261,9 +1261,15 @@ templates used by the :class:`ModelAdmin` views:
     filtering based on add, change, and delete permissions::
 
         class MyModelAdmin(admin.ModelAdmin):
+            inlines = (MyInline,)
+
             def get_inline_instances(self, request, obj=None):
                 return [inline(self.model, self.admin_site) for inline in self.inlines]
 
+    If you override this method, make sure that the returned inlines are
+    instances of the classes defined in :attr:`inlines` or you might encounter
+    a "Bad Request" error when adding related objects.
+
 .. method:: ModelAdmin.get_urls()
 
     The ``get_urls`` method on a ``ModelAdmin`` returns the URLs to be used for