Clarify python_2_unicode_compatible is for classes.

six.py

@@ -922,7 +922,7 @@ def ensure_text(s, encoding='utf-8', errors='strict'):
 
 def python_2_unicode_compatible(klass):
     """
-    A decorator that defines __unicode__ and __str__ methods under Python 2.
+    A class decorator that defines __unicode__ and __str__ methods under Python 2.
     Under Python 3 it does nothing.
 
     To support Python 2 and 3 with a single code base, define a __str__ method