Rephrase a few comments for clarity.

I was confused by what "intended to be parallel serially" meant, until
Robert Haas and David G. Johnston explained it. Rephrase the comment to
make it more clear, using David's suggested wording.

Discussion: https://www.postgresql.org/message-id/1fec9022-41e8-e484-70ce-2179b08c2092%40iki.fi

src/backend/executor/nodeSeqscan.c

@@ -65,8 +65,8 @@ SeqNext(SeqScanState *node)
 	if (scandesc == NULL)
 	{
 		/*
-		 * We reach here if the scan is not parallel, or if we're executing a
-		 * scan that was intended to be parallel serially.
+		 * We reach here if the scan is not parallel, or if we're serially
+		 * executing a scan that was planned to be parallel.
 		 */
 		scandesc = heap_beginscan(node->ss.ss_currentRelation,
 								  estate->es_snapshot,