As Helen has said, these are 2 separate notes and she is (mostly) correct in both.
While "Make & Mend" does mean crews making, repairing or cleaning their kit (not just clothes), it also refers to time spent doing other activities other than 'work' but not time ashore or 'leave'. This can be sport, studying for promotion, writing letters home, or any other beneficial task.
'Detention Bks' does mean Detention Barracks (aka Detention Quarters - DQs) and was (still is) far worse than any civilian prison. It was also unusual to return to a ship having served time in one as their punishment was reserved for the more serious crimes. Usually personnel leaving Detention Barracks were dishonourably discharged from the Service.