SF bug #706546: u''.translate not documented

Clarified the difference between translate methods for string objects and
Unicode objects.

Doc/lib/libstring.tex

@@ -276,7 +276,17 @@ parameter cannot be passed in earlier 2.2 versions]{2.2.3}
   Delete all characters from \var{s} that are in \var{deletechars} (if 
   present), and then translate the characters using \var{table}, which 
   must be a 256-character string giving the translation for each
-  character value, indexed by its ordinal.  
+  character value, indexed by its ordinal.
+
+  For Unicode objects, the \method{translate()} method does not
+  accept the optional \var{deletechars} argument.  Instead, it
+  returns a copy of the \var{s} where all characters have been mapped
+  through the given translation table which must be a mapping of
+  Unicode ordinals to Unicode ordinals, Unicode strings or \code{None}.
+  Unmapped characters are left untouched. Characters mapped to \code{None}
+  are deleted.  Note, a more flexible approach is to create a custom
+  character mapping codec using the \refmodule{codecs} module (see
+  \module{encodings.cp1251} for an example).
 \end{funcdesc}
 
 \begin{funcdesc}{upper}{s}