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

src/backend/replication/logical/reorderbuffer.c

@@ -3500,8 +3500,8 @@ ReorderBufferCheckMemoryLimit(ReorderBuffer *rb)
 	while (rb->size >= logical_decoding_work_mem * 1024L)
 	{
 		/*
-		 * Pick the largest transaction (or subtransaction) and evict it from
-		 * memory by streaming, if possible.  Otherwise, spill to disk.
+		 * Pick the largest transaction and evict it from memory by streaming,
+		 * if possible.  Otherwise, spill to disk.
 		 */
 		if (ReorderBufferCanStartStreaming(rb) &&
 			(txn = ReorderBufferLargestTopTXN(rb)) != NULL)