some timezone doc improvements (closes #16667)

From the combined efforts of Berber Peksage, Andrew Svetlov, and Yayoi Ukai.

Doc/library/datetime.rst

@@ -1687,11 +1687,11 @@ only EST (fixed offset -5 hours), or only EDT (fixed offset -4 hours)).
 .. seealso::
 
    `pytz <http://pypi.python.org/pypi/pytz/>`_
-      The standard library has no :class:`tzinfo` instances except for UTC, but
-      there exists a third-party library which brings the *IANA timezone
-      database* (also known as the Olson database) to Python: *pytz*.
+      The standard library has :class:`timezone` class for handling arbitrary
+      fixed offsets from UTC and :attr:`timezone.utc` as UTC timezone instance.
 
-      *pytz* contains up-to-date information and its usage is recommended.
+      *pytz* library brings the *IANA timezone database* (also known as the
+      Olson database) to Python and its usage is recommended.
 
    `IANA timezone database <http://www.iana.org/time-zones>`_
       The Time Zone Database (often called tz or zoneinfo) contains code and
@@ -1728,6 +1728,8 @@ made to civil time.
   *offset*, HH and MM are two digits of ``offset.hours`` and
   ``offset.minutes`` respectively.
 
+  .. versionadded:: 3.2
+
 .. method:: timezone.utcoffset(dt)
 
   Return the fixed value specified when the :class:`timezone` instance is