Update comment in ExecInsert() regarding batch insertion.

Remove the stale text that is a leftover from an earlier version of the
patch to add support for batch insertion, and adjust the wording in the
remaining text.

Back-patch to v14 where batch insertion came in.

Review and wording adjustment by Tom Lane.

Discussion: https://postgr.es/m/CAPmGK14goatHPHQv2Aeu_UTKqZ%2BBO%2BP%2Bzd3HKv5D%2BdyyfWKDSw%40mail.gmail.com

src/backend/executor/nodeModifyTable.c

@@ -679,9 +679,8 @@ ExecInsert(ModifyTableState *mtstate,
 		if (resultRelInfo->ri_BatchSize > 1)
 		{
 			/*
-			 * If a certain number of tuples have already been accumulated, or
-			 * a tuple has come for a different relation than that for the
-			 * accumulated tuples, perform the batch insert
+			 * When we've reached the desired batch size, perform the
+			 * insertion.
 			 */
 			if (resultRelInfo->ri_NumSlots == resultRelInfo->ri_BatchSize)
 			{