Add docs and reduction aggregations

java/src/main/java/ai/rapids/cudf/GroupByAggregation.java

@@ -316,10 +316,24 @@ public static GroupByAggregation mergeTDigest(int delta) {
     return new GroupByAggregation(Aggregation.mergeTDigest(delta));
   }
 
+  /**
+   * Histogram aggregation, computing the frequencies for each unique row.
+   *
+   * A histogram is given as a lists column, in which the first child stores unique rows from
+   * the input values and the second child stores their corresponding frequencies.
+   *
+   * @return A lists of structs column in which each list contains a histogram corresponding to
+   *         an input key.
+   */
   public static GroupByAggregation histogram() {
     return new GroupByAggregation(Aggregation.histogram());
   }
 
+  /**
+   * MergeHistogram aggregation, to merge multiple histograms.
+   *
+   * @return A new histogram in which the frequencies of the unique rows are sum up.
+   */
   public static GroupByAggregation mergeHistogram() {
     return new GroupByAggregation(Aggregation.mergeHistogram());
   }

java/src/main/java/ai/rapids/cudf/ReductionAggregation.java

@@ -286,4 +286,22 @@ public static ReductionAggregation mergeSets(NullEquality nullEquality, NaNEqual
     return new ReductionAggregation(Aggregation.mergeSets(nullEquality, nanEquality));
   }
 
+  /**
+   * Create HistogramAggregation, computing the frequencies for each unique row.
+   *
+   * @return A structs column in which the first child stores unique rows from the input and the
+   *         second child stores their corresponding frequencies.
+   */
+  public static ReductionAggregation histogram() {
+    return new ReductionAggregation(Aggregation.histogram());
+  }
+
+  /**
+   * Create MergeHistogramAggregation, to merge multiple histograms.
+   *
+   * @return A new histogram in which the frequencies of the unique rows are sum up.
+   */
+  public static ReductionAggregation mergeHistogram() {
+    return new ReductionAggregation(Aggregation.mergeHistogram());
+  }
 }