Fix comment for max_cached_tuplebufs definition

Hello Hackers,

While working on some issue in logical decoding, I found some
inconsistencies in the comment for defining max_cached_tuplebufs in
reorderbuffer.c. It only exists till PG10 because after that the
definition got removed by the generational memory allocator patch. The
variable is defined as follows in reorderbuffer.c:
static const Size max_cached_tuplebufs = 4096 * 2;  /* ~8MB */

And it gets compared with rb->nr_cached_tuplebufs in
ReorderBufferReturnTupleBuf as follows:
if (tuple->alloc_tuple_size == MaxHeapTupleSize &&
    rb->nr_cached_tuplebufs < max_cached_tuplebufs)

                                   {
    rb->nr_cached_tuplebufs++;
}

So, what this variable actually tracks is 4096 * 2 times
MaxHeapTupleSize amount of memory which is approximately 64MB. I've
attached a patch to modify the comment.

But, I'm not sure whether the intention was to keep 8MB cache only. In
that case, I can come up with another patch.

Thoughts?

-- 
Thanks & Regards,
Kuntal Ghosh
EnterpriseDB: http://www.enterprisedb.com