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

Currently, RDD function aggregate's parameter doesn't explain well, especially parameter "zeroValue".
It's helpful to let junior scala user know that "zeroValue" attend both "seqOp" and "combOp" phase.

Author: Tommy YU <tummyyu@163.com>

Closes #10587 from Wenpei/rdd_aggregate_doc.

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

@@ -970,6 +970,13 @@ 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
@@ -988,6 +995,13 @@ 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