[Doc] Fix docstring of aten::Sort (dmlc#2579)

A small patch to dmlc#2391.

include/dgl/aten/array_ops.h

@@ -314,16 +314,12 @@ IdArray NonZero(NDArray array);
  * is always in int64.
  *
  * \param array Input array.
- * \param num_bits The number of bits used by the range of values in the array,
- * or 0 to use all bits of the type. This is currently only used when sort
- * arrays on the GPU. 
- * \param num_bits The number of bits used in key comparison. The bits are
- * right aligned. For example, setting `num_bits` to 8 means using bits from
- * `sizeof(IdType) * 8 - num_bits` (inclusive) to `sizeof(IdType) * 8`
- * (exclusive). Setting it to a small value could speed up the sorting if the
- * underlying sorting algorithm is radix sort (e.g., on GPU). Setting it to
- * value of zero, uses full number of bits of the type (sizeof(IdType)*8).
- * On CPU, it currently has no effect.
+ * \param num_bits The number of bits used in key comparison. For example, if the data type
+ *                 of the input array is int32_t and `num_bits = 8`, it only uses bits in index
+ *                 range [0, 8) for sorting. Setting it to a small value could
+ *                 speed up the sorting if the underlying sorting algorithm is radix sort (e.g., on GPU).
+ *                 Setting it to zero (default value) means using all the bits for comparison.
+ *                 On CPU, it currently has no effect.
  * \return A pair of arrays: sorted values and sorted index to the original position.
  */
 std::pair<IdArray, IdArray> Sort(IdArray array, int num_bits = 0);