Updated a couple admonitions to use the warning directive.

docs/ref/unicode.txt

@@ -68,7 +68,7 @@ Python 2 with unicode literals or Python 3::
 
 See also :doc:`Python 3 compatibility </topics/python3>`.
 
-.. admonition:: Warning
+.. warning::
 
     A bytestring does not carry any information with it about its encoding.
     For that reason, we have to make an assumption, and Django assumes that all

docs/topics/auth/customizing.txt

@@ -408,7 +408,7 @@ This dotted pair describes the name of the Django app (which must be in your
 :setting:`INSTALLED_APPS`), and the name of the Django model that you wish to
 use as your User model.
 
-.. admonition:: Warning
+.. warning::
 
    Changing :setting:`AUTH_USER_MODEL` has a big effect on your database
    structure. It changes the tables that are available, and it will affect the