Added comment to why __unicode__ goes after __str__.

--HG--
branch : trunk
diff --git a/jinja2/environment.py b/jinja2/environment.py
index 965b058..fe7ed02 100644
--- a/jinja2/environment.py
+++ b/jinja2/environment.py
@@ -814,6 +814,10 @@
     def __str__(self):
         return unicode(self).encode('utf-8')
 
+    # unicode goes after __str__ because we configured 2to3 to rename
+    # __unicode__ to __str__.  because the 2to3 tree is not designed to
+    # remove nodes from it, we leave the above __str__ around and let
+    # it override at runtime.
     def __unicode__(self):
         return concat(self._body_stream)