documentation: add missing documentation of archive_preserve options

btrbk.conf.example

@@ -46,9 +46,13 @@ snapshot_dir               _btrbk_snap
 # Preserve all backup targets for a minimum period of time.
 #target_preserve_min        no
 
-# Preserve matrix for backup targets:
+# Retention policy for backup targets:
 #target_preserve            <NN>h <NN>d <NN>w <NN>m <NN>y
 
+# Retention policy for archives ("btrbk archive" command):
+#archive_preserve_min       no
+#archive_preserve           <NN>h <NN>d <NN>w <NN>m <NN>y
+
 # Specify SSH private key for "ssh://" volumes / targets:
 #ssh_identity               /etc/btrbk/ssh/id_ed25519
 #ssh_user                   root
@@ -107,6 +111,9 @@ snapshot_preserve       14d
 target_preserve_min     no
 target_preserve         20d 10w *m
 
+archive_preserve_min    latest
+archive_preserve        12m 10y
+
 # Backup to external disk mounted on /mnt/btr_backup
 volume /mnt/btr_pool
   # no action if external disk is not attached
@@ -116,7 +123,7 @@ volume /mnt/btr_pool
     target send-receive      /mnt/btr_backup/_btrbk
 
   subvolume  kvm
-    # use different preserve matrix for kvm backups
+    # use different retention policy for kvm backups
     target_preserve          7d 4w
     target send-receive      /mnt/btr_backup/_btrbk
 

doc/btrbk.conf.5

@@ -178,6 +178,16 @@ backup only). If set to \[lq]no\[rq], only the backups following the
 \fItarget_preserve\fR policy are created. Defaults to \[lq]all\[rq].
 .RE
 .PP
+\fBarchive_preserve\fR  no|<retention_policy>
+.PD 0
+.PP
+\fBarchive_preserve_min\fR  all|latest|no|<number>{h,d,w,m,y}
+.PD
+.RS 4
+Set retention policy for archives ("btrbk archive" command), with same
+semantics as \fItarget_preserve\fR, \fItarget_preserve_min\fR.
+.RE
+.PP
 \fBpreserve_day_of_week\fR  monday|tuesday|...|sunday
 .RS 4
 Defines on what day a backup/snapshot is considered as a weekly