Clarify note in BZ2File docs about lack of multi-stream support (issue #1625).

Doc/library/bz2.rst

@@ -70,8 +70,11 @@ Handling of compressed files is offered by the :class:`BZ2File` class.
       This class does not support input files containing multiple streams (such
       as those produced by the :program:`pbzip2` tool). When reading such an
       input file, only the first stream will be accessible. If you require
-      support for multi-stream files, consider using the third-party `bz2file
-      module <http://pypi.python.org/pypi/bz2file>`_ instead of this class.
+      support for multi-stream files, consider using the third-party
+      :mod:`bz2file` module (available from
+      `PyPI <http://pypi.python.org/pypi/bz2file>`_). This module provides a
+      backport of Python 3.3's :class:`BZ2File` class, which does support
+      multi-stream files.
 
 
    .. method:: close()