Fix comments in heapam.c.

After commits 85f6b49 and 3ba59cc, we can allow parallel inserts
which was earlier not possible as parallel group members won't conflict
for relation extension and page lock.  In those commits, we forgot to
update comments at few places.

Author: Amit Kapila
Reviewed-by: Robert Haas and Dilip Kumar
Backpatch-through: 13
Discussion: https://postgr.es/m/CAFiTN-tMrQh5FFMPx5aWJ+1gi1H6JxktEhq5mDwCHgnEO5oBkA@mail.gmail.com

Author: Amit Kapila

src/backend/access/heap/heapam.c

@@ -2013,12 +2013,10 @@ heap_prepare_insert(Relation relation, HeapTuple tup, TransactionId xid,
 					CommandId cid, int options)
 {
 	/*
-	 * Parallel operations are required to be strictly read-only in a parallel
-	 * worker.  Parallel inserts are not safe even in the leader in the
-	 * general case, because group locking means that heavyweight locks for
-	 * relation extension or GIN page locks will not conflict between members
-	 * of a lock group, but we don't prohibit that case here because there are
-	 * useful special cases that we can safely allow, such as CREATE TABLE AS.
+	 * To allow parallel inserts, we need to ensure that they are safe to be
+	 * performed in workers. We have the infrastructure to allow parallel
+	 * inserts in general except for the cases where inserts generate a new
+	 * CommandId (eg. inserts into a table having a foreign key column).
 	 */
 	if (IsParallelWorker())
 		ereport(ERROR,
@@ -5694,10 +5692,10 @@ heap_inplace_update(Relation relation, HeapTuple tuple)
 	uint32		newlen;
 
 	/*
-	 * For now, parallel operations are required to be strictly read-only.
-	 * Unlike a regular update, this should never create a combo CID, so it
-	 * might be possible to relax this restriction, but not without more
-	 * thought and testing.  It's not clear that it would be useful, anyway.
+	 * For now, we don't allow parallel updates.  Unlike a regular update,
+	 * this should never create a combo CID, so it might be possible to relax
+	 * this restriction, but not without more thought and testing.  It's not
+	 * clear that it would be useful, anyway.
 	 */
 	if (IsInParallelMode())
 		ereport(ERROR,