cleaned up GroupBy description

cpp/src/arrow/compute/exec/groupby.h

@@ -40,23 +40,24 @@ struct SimpleAggregate {
   std::shared_ptr<FunctionOptions> options;
 };
 
-/// Helper function to perform a group-by given the arguments and keys as columns instead
-/// of an exec batch / record batch.
+/// Convenience function to perform a group-by given the arguments and keys as columns
 ///
-/// This will combine the inputs into a single exec batch, feed that exec batch into a
-/// plan with an aggregate node, and then extract the result of that plan into record
-/// batches.
+/// The result will be calculated using an exec plan with an aggregate node
 ///
-/// The output may contain multiple exec batches if the input is large enough to be
+/// The output may contain multiple chunks if the input is large enough to be
 /// processed in pieces
 ///
-/// The aggregates array must be the same size as the arguments array.  Each aggregate
-/// will be applied to the argument with the same index.
+/// The aggregates vector must be the same size as the arguments vector.  Each aggregate
+/// will be applied to the argument with the same index.  An array could be included in
+/// the `arguments` vector multiple times to compute multiple aggregates.
 ///
-/// The resulting batches will have one column for each key and one column for each
-/// aggregate.
+/// \return a table that will have one column for each aggregate, named after they
+/// aggregate function, and one column for each key, named key0, key1, ...
 ///
-/// The output columns will be named for the aggregate function names
+/// If there are no arguments/aggregates then the returned table will have one row
+/// for each unique combination of keys
+///
+/// If there are no keys then the aggregates will be applied to the full array
 ARROW_EXPORT
 Result<std::shared_ptr<Table>> GroupBy(
     const std::vector<std::shared_ptr<Array>>& arguments,