diff -r cb44fef5ea1d Doc/library/stdtypes.rst
--- a/Doc/library/stdtypes.rst	Thu Jul 21 01:11:30 2011 +0200
+++ b/Doc/library/stdtypes.rst	Thu Jul 21 07:19:01 2011 +0300
@@ -1404,7 +1404,10 @@
 
 .. method:: str.upper()
 
-   Return a copy of the string converted to uppercase.
+   Return a copy of the string converted to uppercase.  If a character has no
+   uppercase, it is left unchanged.  Note that ``str.upper().isupper()`` might
+   be ``False`` if the char is left unchanged or if its Unicode category is not
+   Lu (Letter, uppercase), but e.g. Lt (Letter, titlecase).
 
 
 .. method:: str.zfill(width)