document KUDU-2671: Change hash number for range partitioning

KUDU-2671 implements that each RANGE partition could span across a different number of HASH partitions. This is very useful if you are managing data retention with RANGE partitions, but your data is growing/varying as time passes, making the data distribution uneven across partitions.

However, I've only found this feature documented in Cloudera's docs (which I rarely use):

https://docs.cloudera.com/cdp-private-cloud-base/7.1.9/impala-reference/topics/impala-kudu-partitioning.html#pnavId3

I have found no mention of this feature in the official docs here:

https://kudu.apache.org/docs/kudu_impala_integration.html#partitioning_tables

nor here:

https://kudu.apache.org/docs/schema_design.html#partitioning