update prune documentation for new --keep-within intervals

docs/usage/prune.rst.inc

@@ -144,10 +144,11 @@ from different machines) in one shared repository, use one prune call per
 series.
 
 The ``--keep-within`` option takes an argument of the form "<int><char>",
-where char is "H", "d", "w", "m", "y". For example, ``--keep-within 2d`` means
+where char is "y", "m", "w", "d", "H", "M", or "S" and represents years, months, 
+weeks, days, hours, minutes or seconds.  For example, ``--keep-within 2d`` means
 to keep all archives that were created within the past 48 hours.
-"1m" is taken to mean "31d". The archives kept with this option do not
-count towards the totals specified by any other options.
+"1m" is taken to mean "31d" and "1y" represents "365d". The archives kept with 
+this option do not count towards the totals specified by any other options.
 
 A good procedure is to thin out more and more the older your backups get.
 As an example, ``--keep-daily 7`` means to keep the latest backup on each day,
@@ -173,4 +174,4 @@ backup archive in the same second).
 
 You can influence how the ``--list`` output is formatted by using the ``--short``
 option (less wide output) or by giving a custom format using ``--format`` (see
-the ``borg repo-list`` description for more details about the format string).
+the ``borg repo-list`` description for more details about the format string).