bpo-33487: improve BZ2File Deprecation and documentation. (GH-6785)

Emit warning when None passed explicitly, list Python version since
deprecation in warning message and docs.
diff --git a/Doc/library/bz2.rst b/Doc/library/bz2.rst
index d5f6225..946cc67 100644
--- a/Doc/library/bz2.rst
+++ b/Doc/library/bz2.rst
@@ -81,7 +81,7 @@
    If *filename* is a file object (rather than an actual file name), a mode of
    ``'w'`` does not truncate the file, and is instead equivalent to ``'a'``.
 
-   The *buffering* argument is ignored. Its use is deprecated.
+   The *buffering* argument is ignored. Its use is deprecated since Python 3.0.
 
    If *mode* is ``'w'`` or ``'a'``, *compresslevel* can be a number between
    ``1`` and ``9`` specifying the level of compression: ``1`` produces the
@@ -109,6 +109,10 @@
 
       .. versionadded:: 3.3
 
+
+   .. deprecated:: 3.0
+      The keyword argument *buffering* was deprecated and is now ignored.
+
    .. versionchanged:: 3.1
       Support for the :keyword:`with` statement was added.