[3.0.x] Added note about incomplete HTML in tutorial 3.

Backport of 26be53dd89 from master

docs/intro/tutorial03.txt

@@ -214,6 +214,13 @@ Put the following code in that template:
         <p>No polls are available.</p>
     {% endif %}
 
+.. note::
+
+    To make the tutorial shorter, all template examples use incomplete HTML. In
+    your own projects you should use `complete HTML documents`__.
+
+__ https://developer.mozilla.org/en-US/docs/Learn/HTML/Introduction_to_HTML/Getting_started#Anatomy_of_an_HTML_document
+
 Now let's update our ``index`` view in ``polls/views.py`` to use the template:
 
 .. code-block:: python