Document that basestring has been replaced by str. Issue #1931.

Doc/whatsnew/3.0.rst

@@ -154,6 +154,9 @@ Strings and Bytes
 * There is only one string type; its name is ``str`` but its behavior
   and implementation are more like ``unicode`` in 2.x.
 
+* The ``basestring`` superclass has been removed. The ``2to3`` tool
+  replaces every occurence of ``basestring`` with ``str``.
+
 * PEP 3137: There is a new type, ``bytes``, to represent binary data
   (and encoded text, which is treated as binary data until you decide
   to decode it).  The ``str`` and ``bytes`` types cannot be mixed; you