[dict-proto] update comment describing layout of ordered hash tables

The comment at the beginning of ordered-hash-table.h desribing the
layout of ordered hash tables seems to have become outdated.

1. There was a "kPrefixSize + " missing in the description of the left
  bound of the first non-fixed-sized part in normal and obsoleted
  tables.

The following only affects obsoleted tables:

2. The bucket count doesn't reside at [kPrefixSize + 0], instead it is
  unchanged w.r.t normal tables, and resides at [kPrefixSize + 2]
3. The next newer table takes place of the element count, residing
  at  [kPrefixSize + 0]. This can be seen by NextTableIndex() being
  defined as NumberOfElementsIndex()
4. The number of removed holes is never written anywhere when
  obsoleting a table. Instead, the number of deleted elements count is
  used for this. This can be see by
  DCHECK_EQ(table->NumberOfDeletedElements(), removed_holes_index);
  in method Rehash
5. kClearedTableSentinel is written to the number of deleted element
  slot when clearing a table, as seen by
  table->SetNumberOfDeletedElements(kClearedTableSentinel);
  in method Clear

Bug: v8:7569, v8:11074
Change-Id: I0e6750fc57c90b6c09fc41f001cd555f7c455fd3
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2505763
Commit-Queue: Frank Emrich <emrich@google.com>
Reviewed-by: Igor Sheludko <ishell@chromium.org>
Cr-Commit-Position: refs/heads/master@{#70918}

src/objects/ordered-hash-table.h

@@ -35,7 +35,7 @@ namespace internal {
 //   [kPrefixSize]: element count
 //   [kPrefixSize + 1]: deleted element count
 //   [kPrefixSize + 2]: bucket count
-//   [kPrefixSize + 3..(3 + NumberOfBuckets() - 1)]: "hash table",
+//   [kPrefixSize + 3..(kPrefixSize + 3 + NumberOfBuckets() - 1)]: "hash table",
 //                            where each item is an offset into the
 //                            data table (see below) where the first
 //                            item in this bucket is stored.
@@ -53,13 +53,15 @@ namespace internal {
 //
 // Memory layout for obsolete table:
 //   [0] : Prefix
-//   [kPrefixSize + 0]: bucket count
-//   [kPrefixSize + 1]: Next newer table
-//   [kPrefixSize + 2]: Number of removed holes or -1 when the table was
-//                      cleared.
-//   [kPrefixSize + 3..(3 + NumberOfRemovedHoles() - 1)]: The indexes
-//                      of the removed holes.
-//   [kPrefixSize + 3 + NumberOfRemovedHoles()..length]: Not used
+//   [kPrefixSize + 0]: Next newer table
+//   [kPrefixSize + 1]: deleted element count or kClearedTableSentinel if
+//                      the table was cleared
+//   [kPrefixSize + 2]: bucket count
+//   [kPrefixSize + 3..(kPrefixSize + 3 + NumberOfDeletedElements() - 1)]:
+//                      The indexes of the removed holes. This part is only
+//                      usable for non-cleared tables, as clearing removes the
+//                      deleted elements count.
+//   [kPrefixSize + 3 + NumberOfDeletedElements()..length]: Not used
 template <class Derived, int entrysize>
 class OrderedHashTable : public FixedArray {
  public: