gh-82907: Document mtime=0 for reproducible tarfile gzip output

tarfile.open() already writes a fixed zero timestamp in the gzip header
when mtime=0 is passed, on both the 'w:gz' and 'w|gz' paths, producing
output that does not depend on the archive creation time. Document this
special value, matching the existing wording in the gzip module docs.

Author: omkar-334

Doc/library/tarfile.rst

@@ -144,7 +144,9 @@ Some facts and figures:
 
    For modes ``'w:gz'`` and ``'w|gz'``, :func:`tarfile.open` accepts the
    keyword argument *mtime* to create a gzip archive header with that mtime. By
-   default, the mtime is set to the time of creation of the archive.
+   default, the mtime is set to the time of creation of the archive. Use
+   *mtime* ``0`` to generate a compressed stream that does not depend on
+   creation time, for reproducible output.
 
    For special purposes, there is a second format for *mode*:
    ``'filemode|[compression]'``.  :func:`tarfile.open` will return a :class:`TarFile`

Misc/NEWS.d/next/Documentation/2026-05-22-13-02-35.gh-issue-82907.N0Rkfa.rst

@@ -0,0 +1,3 @@
+Document that passing ``mtime=0`` to :func:`tarfile.open` produces a
+gzip-compressed archive whose output does not depend on creation time, for
+reproducible output.