Added some text I meant to add in the last commit.

docs/howto/upgrade-version.txt

@@ -50,7 +50,7 @@ Once you're ready, it is time to :doc:`install the new Django version
 might want to set up a new environment will all the dependencies first.
 
 Exactly which steps you will need to take depends on your installation process.
-The most convenient way is to use pip_:
+The most convenient way is to use pip_ with the ``--upgrade`` or ``-U`` flag:
 
 .. code-block:: bash