Minor comment updates

Fix a couple of typos, and update a comment about why we set a BMS to
NULL.

Author: David Rowley
Discussion: http://postgr.es/m/CAKJS1f-tux=KdUz6ENJ9GHM_V2qgxysadYiOyQS9Ko9PTteVhQ@mail.gmail.com

src/backend/executor/execPartition.c

@@ -1524,9 +1524,12 @@ ExecFindInitialMatchingSubPlans(PartitionPruneState *prunestate, int nsubnodes)
 
 	/*
 	 * Record that partition pruning has been performed for external params.
-	 * This partly also serves to ensure we never call this function twice
-	 * with the same input and also so that ExecFindMatchingSubPlans is aware
-	 * that pruning has already been performed for external Params.
+	 * These are not required again afterwards, and nullifying them helps
+	 * ensure nothing accidentally calls this function twice on the same
+	 * PartitionPruneState.
+	 *
+	 * (Note we keep prunestate->allparams, because we do use that one
+	 * repeatedly in ExecFindMatchingSubPlans).
 	 */
 	bms_free(prunestate->extparams);
 	prunestate->extparams = NULL;
@@ -1607,7 +1610,7 @@ ExecFindInitialMatchingSubPlans(PartitionPruneState *prunestate, int nsubnodes)
 
 /*
  * ExecFindMatchingSubPlans
- *		Determine which subplans match the the pruning steps detailed in
+ *		Determine which subplans match the pruning steps detailed in
  *		'pprune' for the current Param values.
  *
  * Here we utilize both external and exec Params for pruning.

src/include/partitioning/partprune.h

@@ -40,7 +40,7 @@ typedef struct PartitionPruneContext
 
 	/*
 	 * Can be set when the context is used from the executor to allow params
-	 * found matching the partition key to be evaulated.
+	 * found matching the partition key to be evaluated.
 	 */
 	PlanState  *planstate;