[heap] Update comments in SlotSet

Bug: chromium:648568
Change-Id: I09ef3911d1817b57d5984ca4121b2579093a1d93
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1722563Reviewed-by: Dominik Inführ <dinfuehr@chromium.org>
Commit-Queue: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#63037}

src/heap/slot-set.h

@@ -52,9 +52,6 @@ class SlotSet : public Malloced {
   void SetPageStart(Address page_start) { page_start_ = page_start; }
 
   // The slot offset specifies a slot at address page_start_ + slot_offset.
-  // This method should only be called on the main thread because concurrent
-  // allocation of the bucket is not thread-safe.
-  //
   // AccessMode defines whether there can be concurrent access on the buckets
   // or not.
   template <AccessMode access_mode = AccessMode::ATOMIC>
@@ -181,7 +178,10 @@ class SlotSet : public Malloced {
   // Iterate over all slots in the set and for each slot invoke the callback.
   // If the callback returns REMOVE_SLOT then the slot is removed from the set.
   // Returns the new number of slots.
-  // This method should only be called on the main thread.
+  //
+  // Iteration can be performed concurrently with other operations that use
+  // atomic access mode such as insertion and removal. However there is no
+  // guarantee about ordering and linearizability.
   //
   // Sample usage:
   // Iterate([](MaybeObjectSlot slot) {