In gzip.rst, extend explanation of argument mtime, and simplify description of attribute mtime

Author: jwuttke

Doc/library/gzip.rst

@@ -100,10 +100,12 @@ The module defines the following items:
    compression, and ``9`` is slowest and produces the most compression. ``0``
    is no compression. The default is ``9``.
 
-   The *mtime* argument is an optional numeric timestamp to be written to
-   the last modification time field in the stream when compressing.  It
-   should only be provided in compression mode.  If omitted or ``None``, the
-   current time is used.  See the :attr:`mtime` attribute for more details.
+   The optional *mtime* argument is the timestamp requested by gzip. The time
+   is in Unix format, i.e., seconds since 00:00:00 GMT, Jan.  1, 1970.
+   If mtime is omitted or None, the current time is used. Use mtime = 0
+   to generate a compressed stream that does not depend on creation time.
+
+   See below for the :attr:`mtime` attribute that is set when decompressing.
 
    Calling a :class:`GzipFile` object's :meth:`close` method does not close
    *fileobj*, since you might wish to append more material after the compressed
@@ -133,15 +135,10 @@ The module defines the following items:
 
    .. attribute:: mtime
 
-      When decompressing, the value of the last modification time field in
-      the most recently read header may be read from this attribute, as an
-      integer.  The initial value before reading any headers is ``None``.
-
-      All :program:`gzip` compressed streams are required to contain this
-      timestamp field.  Some programs, such as :program:`gunzip`\ , make use
-      of the timestamp.  The format is the same as the return value of
-      :func:`time.time` and the :attr:`~os.stat_result.st_mtime` attribute of
-      the object returned by :func:`os.stat`.
+      When decompressing, this attribute is set to last timestamp in the most
+      recently read header.  It is an integer, holding the number of seconds
+      since the Unix epoch (00:00:00 GMT, Jan. 1, 1970).
+      The initial value before reading any headers is ``None``.
 
    .. versionchanged:: 3.1
       Support for the :keyword:`with` statement was added, along with the
@@ -275,4 +272,3 @@ Command line options
 .. cmdoption:: -h, --help
 
    Show the help message.
-