Merge from 3.3: Improve str() and object.__str__() docs (issue #13538).

diff --cc Doc/reference/datamodel.rst
Simple merge

diff --cc Lib/test/test_unicode.py
Simple merge

diff --cc Misc/NEWS
index f27ce2e3f6a03ddadb7403123f856a9c1232e702,b3dacd2ab4c58ef2f87d9c95b4323626b13f9872..a56194c1097157492bd7822826217dbb475d59df
--- 1/Misc/NEWS
--- 2/Misc/NEWS
+++ b/Misc/NEWS
@@@ -395,8 -277,16 +395,10 @@@ Buil
  Documentation
  -------------
  
 -- Issue #16489: Make it clearer that importlib.find_loader() requires any and
 -  all packages to be separately imported.
+ - Issue #13538: Improve str() and object.__str__() documentation.
+ 
 +- Issue #16489: Make it clearer that importlib.find_loader() needs parent
 +  packages to be explicitly imported.
  
  - Issue #16400: Update the description of which versions of a given package
    PyPI displays.