tape: improve retention period docu

docs/tape-backup.rst

@@ -384,6 +384,9 @@ one media pool, so a job only uses tapes from that pool.
      trigger new set creation by running a backup job with the
      ``--export`` option.
 
+     .. NOTE:: Retention period starts with the existence of a newer
+	media set.
+
    - Always create a new media set.
 
      With this setting each backup job creates a new media set. This
@@ -394,6 +397,9 @@ one media pool, so a job only uses tapes from that pool.
      size. Small set are easier to handle, you can move sets to an
      off-site vault, and restore is much faster.
 
+     .. NOTE:: Retention period starts with the creation time of the
+        media set.
+
    - Create a new set when the specified Calendar Event triggers.
 
      .. _systemd.time manpage: https://manpages.debian.org/buster/systemd/systemd.time.7.en.html
@@ -406,6 +412,9 @@ one media pool, so a job only uses tapes from that pool.
 
      This balances between space efficency and media count.
 
+     .. NOTE:: Retention period starts when the calendar event
+        triggers.
+
    Additionally, the following events may allocate a new media set:
 
    - Required tape is offline (and you use a tape library).
@@ -418,9 +427,7 @@ one media pool, so a job only uses tapes from that pool.
 
 .. topic:: Retention Policy
 
-   Defines how long we want to keep the data. Retention period starts
+   Defines how long we want to keep the data.
-   with the existence of a newer media set, e.g. when the allocation
-   policy calendar event triggers.
 
    - Always overwrite media.