Issue #25496: Merge tarfile doc from 3.5

Doc/library/tarfile.rst

@@ -101,7 +101,7 @@ Some facts and figures:
 
    For modes ``'w:gz'``, ``'r:gz'``, ``'w:bz2'``, ``'r:bz2'``, ``'x:gz'``,
    ``'x:bz2'``, :func:`tarfile.open` accepts the keyword argument
-   *compresslevel* to specify the compression level of the file.
+   *compresslevel* (default ``9``) to specify the compression level of the file.
 
    For special purposes, there is a second format for *mode*:
    ``'filemode|[compression]'``.  :func:`tarfile.open` will return a :class:`TarFile`