Improve comments for parallel executor estimation functions.

The previous comment (which was copied as boilerplate from one file
to the next) implied that it was the executor node itself which was
being serialized, but that's not right.  We're not serializing
the executor nodes; we're just allowing them to store some
additional information in DSM.  Adjusts the comment to reflect this.

Discussion: http://postgr.es/m/CA+TgmoaHVinxG=3h6qBAsyV8xaDyQwbzK7YZnYfE8nJFMK1=FA@mail.gmail.com

src/backend/executor/nodeBitmapHeapscan.c

@@ -934,7 +934,8 @@ BitmapShouldInitializeSharedState(ParallelBitmapHeapState *pstate)
 /* ----------------------------------------------------------------
  *		ExecBitmapHeapEstimate
  *
- *		estimates the space required to serialize bitmap scan node.
+ *		Compute the amount of space we'll need in the parallel
+ *		query DSM, and inform pcxt->estimator about our needs.
  * ----------------------------------------------------------------
  */
 void

src/backend/executor/nodeIndexonlyscan.c

@@ -604,7 +604,8 @@ ExecInitIndexOnlyScan(IndexOnlyScan *node, EState *estate, int eflags)
 /* ----------------------------------------------------------------
  *		ExecIndexOnlyScanEstimate
  *
- *	estimates the space required to serialize index-only scan node.
+ *		Compute the amount of space we'll need in the parallel
+ *		query DSM, and inform pcxt->estimator about our needs.
  * ----------------------------------------------------------------
  */
 void

src/backend/executor/nodeIndexscan.c

@@ -1644,7 +1644,8 @@ ExecIndexBuildScanKeys(PlanState *planstate, Relation index,
 /* ----------------------------------------------------------------
  *		ExecIndexScanEstimate
  *
- *		estimates the space required to serialize indexscan node.
+ *		Compute the amount of space we'll need in the parallel
+ *		query DSM, and inform pcxt->estimator about our needs.
  * ----------------------------------------------------------------
  */
 void

src/backend/executor/nodeSeqscan.c

@@ -289,7 +289,8 @@ ExecReScanSeqScan(SeqScanState *node)
 /* ----------------------------------------------------------------
  *		ExecSeqScanEstimate
  *
- *		estimates the space required to serialize seqscan node.
+ *		Compute the amount of space we'll need in the parallel
+ *		query DSM, and inform pcxt->estimator about our needs.
  * ----------------------------------------------------------------
  */
 void