C comment: add description of root_tuple_slot

Reported-by: Amit Langote

Discussion: https://postgr.es/m/d2e6674c-c5c6-fe89-1d0b-3534b9db0476@lab.ntt.co.jp

src/include/executor/execPartition.h

@@ -90,6 +90,10 @@ typedef struct PartitionDispatchData *PartitionDispatch;
  *								given leaf partition's rowtype after that
  *								partition is chosen for insertion by
  *								tuple-routing.
+ * root_tuple_slot				TupleTableSlot to be used to transiently hold
+ *								copy of a tuple that's being moved across
+ *								partitions in the root partitioned table's
+ *								rowtype
  *-----------------------
  */
 typedef struct PartitionTupleRouting