Inaccurate comments in ReorderBufferCheckMemoryLimit()

Hi all,

While reading the code, I realized that the following code comments
might not be accurate:

        /*
         * Pick the largest transaction (or subtransaction) and evict it from
         * memory by streaming, if possible.  Otherwise, spill to disk.
         */
        if (ReorderBufferCanStartStreaming(rb) &&
            (txn = ReorderBufferLargestStreamableTopTXN(rb)) != NULL)
        {
            /* we know there has to be one, because the size is not zero */
            Assert(txn && rbtxn_is_toptxn(txn));
            Assert(txn->total_size > 0);
            Assert(rb->size >= txn->total_size);

            ReorderBufferStreamTXN(rb, txn);
        }

AFAICS since ReorderBufferLargestStreamableTopTXN() returns only
top-level transactions, the comment above the if statement is not
right. It would not pick a subtransaction.

Also, I'm not sure that the second comment "we know there has to be
one, because the size is not zero" is right since there might not be
top-transactions that are streamable. I think this is why we say
"Otherwise, spill to disk".

I've attached a patch to fix these comments. Feedback is very welcome.

Regards,

-- 
Masahiko Sawada
Amazon Web Services: https://aws.amazon.com