document the compress_level argument to tarfile.open (closes #21404)

Patch by Katherine Busch.
diff --git a/Doc/library/tarfile.rst b/Doc/library/tarfile.rst
index 1ef2703..15b88f8 100644
--- a/Doc/library/tarfile.rst
+++ b/Doc/library/tarfile.rst
@@ -81,6 +81,10 @@
    If *fileobj* is specified, it is used as an alternative to a :term:`file object`
    opened in binary mode for *name*. It is supposed to be at position 0.
 
+   For modes ``'w:gz'``, ``'r:gz'``, ``'w:bz2'``, ``'r:bz2'``, :func:`tarfile.open`
+   accepts the keyword argument *compresslevel* 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`
    object that processes its data as a stream of blocks.  No random seeking will
diff --git a/Misc/ACKS b/Misc/ACKS
index 9cf4572..571f1a6 100644
--- a/Misc/ACKS
+++ b/Misc/ACKS
@@ -195,6 +195,7 @@
 Alastair Burt
 Tarn Weisner Burton
 Lee Busby
+Katherine Busch
 Ralph Butler
 Nicolas Cadou
 Jp Calderone