[doc] update on NTP clock synchronization
[doc] update on NTP clock synchronization

Updated NTP configuration best practices and clock synchronization
troubleshooting tips.

Change-Id: Ib3b1485b6df846a3286f52003684386e59e972ac
Reviewed-on: http://gerrit.cloudera.org:8080/15141
Tested-by: Kudu Jenkins
Reviewed-by: Adar Dembo <866ca730bfc7f544e3ebce604ceeba4879f23df0@cloudera.com>

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