docs: add warning about overlapping directories
docs: add warning about overlapping directories

There wasn't a warning against having overlapping directories for
different Kudu processes, but there should be since it would cause Kudu
to not start up.

Change-Id: I8905ef81aa6eaa6fbfa5845110b4016a0b56be9b
Reviewed-on: http://gerrit.cloudera.org:8080/12013
Tested-by: Kudu Jenkins
Reviewed-by: Will Berkeley <c74686ab413a71926fc1bc9d16ebfa30e619336a@gmail.com>

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