fix: Clarify comment in backup consumer logic (Aiven-Open#781)

* fix: Clarify comment in backup consumer logic

* Update api.py

karapace/backup/api.py

@@ -299,7 +299,13 @@ def _consume_records(
     if start_offset >= end_offset:
         raise EmptyPartition
 
-    end_offset -= 1  # high watermark to actual end offset
+    # confluent-kafka-python returns end offset + 1, i.e. the value that will
+    # be assigned to the next record to be produced. To get the highest offset
+    # already produced we need to subtract 1. Note that this has little to do
+    # with high watermark, which takes into account the highest record offset
+    # to be fully in sync across ISRs, and therefore can be an arbitrary
+    # number less than or equal to the LEO.
+    end_offset -= 1
 
     while True:
         record: Message | None = consumer.poll(timeout=poll_timeout.seconds)