[doxygen] fix KuduScanTokenBuilder::SetDiffScan (take two)
[doxygen] fix KuduScanTokenBuilder::SetDiffScan (take two)

Since both {KuduScanner,KuduScanTokenBuilder}::SetDiffScan() are both
under the same PRIVATE_API conditional, it's possible to use the
@copydoc to avoid the duplication of the description.

This is a follow-up to 0b4948ea3.

Change-Id: I2de45bf62bd8ef240b0d0e0cd3f887ef3d40cd04
Reviewed-on: http://gerrit.cloudera.org:8080/13841
Tested-by: Kudu Jenkins
Reviewed-by: Grant Henke <4cf7ebbe638391c4d27a10cf751b99bdbd1a1880@apache.org>

helifu/kudu,helifu/kudu,helifu/kudu,helifu/kudu,helifu/kudu,helifu/kudu,helifu/kudu,helifu/kudu,helifu/kudu