docs: add notes for multi master migration
docs: add notes for multi master migration

Change-Id: I7e469bc23b650f3b99c785311acc665ac20f33ea
Reviewed-on: http://gerrit.cloudera.org:8080/5728
Tested-by: Kudu Jenkins
Reviewed-by: Adar Dembo <866ca730bfc7f544e3ebce604ceeba4879f23df0@cloudera.com>

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