Removed distracting note from tutorial 4.

The note on a possible race condition is inappropriate in this
tutorial setting. To quote Diátaxis:

> Your job is to guide the learner to a successful conclusion. There
> may be many interesting diversions along the way … - ignore them.

Co-Authored-By: Ryan Hiebert <ryan@ryanhiebert.com>

docs/intro/tutorial04.txt

@@ -74,6 +74,7 @@ create a real version. Add the following to ``polls/views.py``:
 .. code-block:: python
     :caption: ``polls/views.py``
 
+    from django.db.models import F
     from django.http import HttpResponse, HttpResponseRedirect
     from django.shortcuts import get_object_or_404, render
     from django.urls import reverse
@@ -97,7 +98,7 @@ create a real version. Add the following to ``polls/views.py``:
                 },
             )
         else:
-            selected_choice.votes += 1
+            selected_choice.votes = F("votes") + 1
             selected_choice.save()
             # Always return an HttpResponseRedirect after successfully dealing
             # with POST data. This prevents data from being posted twice if a
@@ -123,6 +124,9 @@ This code includes a few things we haven't covered yet in this tutorial:
   :exc:`KeyError` and redisplays the question form with an error
   message if ``choice`` isn't given.
 
+* ``F("votes") + 1`` :ref:`instructs the database
+  <avoiding-race-conditions-using-f>` to increase the vote count by 1.
+
 * After incrementing the choice count, the code returns an
   :class:`~django.http.HttpResponseRedirect` rather than a normal
   :class:`~django.http.HttpResponse`.
@@ -190,19 +194,6 @@ Now, go to ``/polls/1/`` in your browser and vote in the question. You should se
 results page that gets updated each time you vote. If you submit the form
 without having chosen a choice, you should see the error message.
 
-.. note::
-    The code for our ``vote()`` view does have a small problem. It first gets
-    the ``selected_choice`` object from the database, then computes the new
-    value of ``votes``, and then saves it back to the database. If two users of
-    your website try to vote at *exactly the same time*, this might go wrong:
-    The same value, let's say 42, will be retrieved for ``votes``. Then, for
-    both users the new value of 43 is computed and saved, but 44 would be the
-    expected value.
-
-    This is called a *race condition*. If you are interested, you can read
-    :ref:`avoiding-race-conditions-using-f` to learn how you can solve this
-    issue.
-
 Use generic views: Less code is better
 ======================================