add comment about hashtable sizes, fixes #2830

src/borg/_hashindex.c

@@ -67,6 +67,11 @@ typedef struct {
  * more and more down to 1.1x. this is to avoid huge jumps in memory allocation,
  * like e.g. 4G -> 8G.
  * these values are generated by hash_sizes.py.
+ *
+ * update: no, we don't need primes or w/ big prime factors, we followed some
+ *         incomplete / irrelevant advice here that did not match our use case.
+ *         otoh, for now, we do not need to change the sizes as they do no harm.
+ *         see ticket #2830.
  */
 static int hash_sizes[] = {
     1031, 2053, 4099, 8209, 16411, 32771, 65537, 131101, 262147, 445649,