Issue #8784: Set tarfile default encoding to 'utf-8' on Windows.

Note: file system encoding cannot be None anymore (since r81190, issue #8610).
diff --git a/Doc/library/tarfile.rst b/Doc/library/tarfile.rst
index c2a9143..b1d7361 100644
--- a/Doc/library/tarfile.rst
+++ b/Doc/library/tarfile.rst
@@ -185,8 +185,8 @@
 
 .. data:: ENCODING
 
-   The default character encoding i.e. the value from either
-   :func:`sys.getfilesystemencoding` or :func:`sys.getdefaultencoding`.
+   The default character encoding: ``'utf-8'`` on Windows,
+   :func:`sys.getfilesystemencoding` otherwise.
 
 
 .. seealso::