Clearly describe when POST_BACKUP_SCRIPT is run or not

In default.conf describe in more detail when POST_BACKUP_SCRIPT
is normally run or run as exit task versus when it is not run, cf.
#2735 (comment)

usr/share/rear/conf/default.conf

@@ -3524,11 +3524,14 @@ POST_RECOVERY_COMMANDS=()
 # When a database is running on a filesystem which is included in the backup
 # you may need to stop a database service before backup via PRE_BACKUP_SCRIPT
 # and restart it after the backup was done via POST_BACKUP_SCRIPT.
-# In case of an error during backup (and provided PRE_BACKUP_SCRIPT is set),
+# In case of an error exit during backup and provided PRE_BACKUP_SCRIPT is set,
 # ReaR will run the POST_BACKUP_SCRIPT as exit task (via ReaR's EXIT_TASKS array).
-# This prevents that the database remains stopped in case of an error during backup
-# which also means the database is active after there was an error during backup
+# This prevents that the database remains stopped in case of an error exit during backup
+# which also means the database is active after there was an error exit during backup
 # when the POST_BACKUP_SCRIPT restarts the database unconditioned.
+# When only POST_BACKUP_SCRIPT is set without a PRE_BACKUP_SCRIPT
+# then POST_BACKUP_SCRIPT is not run in case of an error exit during backup.
+# POST_BACKUP_SCRIPT is run if there is some error but no error exit during backup.
 
 # Called at the beginning of the backup part in the mkbackup/mkbackuponly workflow:
 PRE_BACKUP_SCRIPT=