kvserver: fix comment when dropping due to exceeding size

Previously, the comment on the queue incorrectly stated that it removes the
lowest-priority element when exceeding its maximum size. This was misleading
because heap only guarantees that the root is the highest priority, not that
elements are globally ordered. This commit updates the comment to clarify that
the removed element might not be the lowest priority. Ideally, we should drop
the lowest priority element when exceeding queue size, but heap doesn't make
this very easy.

Author: wenyihu6

pkg/kv/kvserver/queue.go

@@ -766,8 +766,11 @@ func (bq *baseQueue) addInternal(
 	item = &replicaItem{rangeID: desc.RangeID, replicaID: replicaID, priority: priority}
 	bq.addLocked(item)
 
-	// If adding this replica has pushed the queue past its maximum size,
-	// remove the lowest priority element.
+	// If adding this replica has pushed the queue past its maximum size, remove
+	// an element. Note that it might not be the lowest priority since heap is not
+	// guaranteed to be globally ordered. Ideally, we would remove the lowest
+	// priority element, but it would require additional bookkeeping or a linear
+	// scan.
 	if pqLen := bq.mu.priorityQ.Len(); pqLen > bq.maxSize {
 		bq.removeLocked(bq.mu.priorityQ.sl[pqLen-1])
 	}