[SPARK-12638] [API DOC] Parameter explanation not very accurate for rdd function "aggregate"

core/src/main/scala/org/apache/spark/rdd/RDD.scala

@@ -1071,6 +1071,13 @@ abstract class RDD[T: ClassTag](
    * apply the fold to each element sequentially in some defined ordering. For functions
    * that are not commutative, the result may differ from that of a fold applied to a
    * non-distributed collection.
+   *
+   * @param zeroValue the initial value for the accumulated result of each partition for the `op`
+   *                  operator, and also the initial value for the combine results from different
+   *                  partitions for the `op` operator - this will typically be the neutral
+   *                  element (e.g. `Nil` for list concatenation or `0` for summation)
+   * @param op an operator used to both accumulate results within a partition and combine results
+   *                  from different partitions
    */
   def fold(zeroValue: T)(op: (T, T) => T): T = withScope {
     // Clone the zero value since we will also be serializing it as part of tasks
@@ -1089,6 +1096,13 @@ abstract class RDD[T: ClassTag](
    * and one operation for merging two U's, as in scala.TraversableOnce. Both of these functions are
    * allowed to modify and return their first argument instead of creating a new U to avoid memory
    * allocation.
+   *
+   * @param zeroValue the initial value for the accumulated result of each partition for the
+   *                  `seqOp` operator, and also the initial value for the combine results from
+   *                  different partitions for the `combOp` operator - this will typically be the
+   *                  neutral element (e.g. `Nil` for list concatenation or `0` for summation)
+   * @param seqOp an operator used to accumulate results within a partition
+   * @param combOp an associative operator used to combine results from different partitions
    */
   def aggregate[U: ClassTag](zeroValue: U)(seqOp: (U, T) => U, combOp: (U, U) => U): U = withScope {
     // Clone the zero value since we will also be serializing it as part of tasks