Fix ReorderBufferCheckMemoryLimit() comment.
Commit
7259736a6 updated the comment but it was not correct since
ReorderBufferLargestStreamableTopTXN() returns only top-level
transactions.
Reviewed-by: Amit Kapila
Discussion: https://postgr.es/m/CAD21AoA9XB7OR86BqvrCe2dMYX%2BZv3-BvVmjF%3DGY2z6jN-kqjg%40mail.gmail.com
Backpatch-through: 14