Linking to both Attic and Borg prune docs from sample config.

sample/config

@@ -7,7 +7,8 @@ repository: user@backupserver:sourcehostname.attic
 
 [retention]
 # Retention policy for how many backups to keep in each category. See
-# https://attic-backup.org/usage.html#attic-prune for details.
+# https://attic-backup.org/usage.html#attic-prune or
+# https://borgbackup.github.io/borgbackup/usage.html#borg-prune for details.
 #keep_within: 3h
 #keep_hourly: 24
 keep_daily: 7