Improve the documentation for `--keep-within`

Move the explanation below the general explanation of the `--keep-*` option
behavior rephrase the last sentence to make it clear that it works like the
other options that were explained in the previous paragraph.

Resolves #7687

docs/usage/prune.rst.inc

@@ -139,12 +139,6 @@ from different machines) in one shared repository, use one prune call per
 data set that matches only the respective archives using the --match-archives
 (-a) option.
 
-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
-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.
-
 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,
 up to 7 most recent days with backups (days without backups do not count).
@@ -158,6 +152,13 @@ minutely, hourly, daily, weekly, monthly, or yearly rules was not otherwise able
 meet its retention target. This enables the first chronological archive to continue
 aging until it is replaced by a newer archive that meets the retention criteria.
 
+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
+to keep all archives that were created within the past 48 hours.
+"1m" is taken to mean "31d". This option is applied before the secondly option
+and like the other options any archives kept by this option do not count towards
+the later rules.
+
 The ``--keep-last N`` option is doing the same as ``--keep-secondly N`` (and it will
 keep the last N archives under the assumption that you do not create more than one
 backup archive in the same second).
@@ -169,4 +170,4 @@ Please note that the "All archives" stats refer to the state after pruning.
 
 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 rlist`` description for more details about the format string).
+the ``borg rlist`` description for more details about the format string).