Add Misc/NEWS entry and relevant doc change for issue 10271.

Doc/library/warnings.rst

@@ -339,8 +339,7 @@ Available Functions
    Write a warning to a file.  The default implementation calls
    ``formatwarning(message, category, filename, lineno, line)`` and writes the
    resulting string to *file*, which defaults to ``sys.stderr``.  You may replace
-   this function with an alternative implementation by assigning to
-   ``warnings.showwarning``.
+   this function with any callable by assigning to ``warnings.showwarning``.
    *line* is a line of source code to be included in the warning
    message; if *line* is not supplied, :func:`showwarning` will
    try to read the line specified by *filename* and *lineno*.

Misc/NEWS

@@ -10,6 +10,8 @@ What's New in Python 3.3 Alpha 1?
 Core and Builtins
 -----------------
 
+- Issue #10271: Allow warnings.showwarning() be any callable.
+
 - Issue #11627: Fix segfault when __new__ on a exception returns a non-exception
   class.