[DOCS] KUDU-2051-Update Impala integration limitations
[DOCS] KUDU-2051-Update Impala integration limitations

- Impala's TIMESTAMP and Kudu's UNIXTIME_MACROS
from the list of limitations.

- Impala now pushes down NULL/NOT NULL to Kudu. Those were
removed from the list.

Change-Id: I884b5c6905bc7371e7696133c53ba0df8594e945
Reviewed-on: http://gerrit.cloudera.org:8080/7355
Reviewed-by: Jean-Daniel Cryans <4bf4c125525b8623ac45dfd7774cbf531df19085@apache.org>
Tested-by: Kudu Jenkins

cloudera/kudu,EvilMcJerkface/kudu,helifu/kudu,InspurUSA/kudu,andrwng/kudu,InspurUSA/kudu,cloudera/kudu,andrwng/kudu,cloudera/kudu,InspurUSA/kudu,EvilMcJerkface/kudu,helifu/kudu,andrwng/kudu,helifu/kudu,InspurUSA/kudu,InspurUSA/kudu,EvilMcJerkface/kudu,EvilMcJerkface/kudu,andrwng/kudu,cloudera/kudu,EvilMcJerkface/kudu,andrwng/kudu,InspurUSA/kudu,andrwng/kudu,cloudera/kudu,EvilMcJerkface/kudu,cloudera/kudu,andrwng/kudu,helifu/kudu,helifu/kudu,andrwng/kudu,andrwng/kudu,helifu/kudu,EvilMcJerkface/kudu,helifu/kudu,cloudera/kudu,InspurUSA/kudu,InspurUSA/kudu,InspurUSA/kudu,helifu/kudu,EvilMcJerkface/kudu,cloudera/kudu,EvilMcJerkface/kudu,cloudera/kudu,helifu/kudu