[docs] Add tip on dealing with planned TS downtime
[docs] Add tip on dealing with planned TS downtime

Rendering available at
https://github.com/wdberkeley/kudu/blob/docfollowerunavailablesec/docs/administration.adoc.

Change-Id: I55a992a00f35945187e02c55594edc6e261a72c4
Reviewed-on: http://gerrit.cloudera.org:8080/11486
Reviewed-by: Andrew Wong <b68e4fdc6430321a6b47400732ff97d7ae91234e@cloudera.com>
Reviewed-by: Grant Henke <4cf7ebbe638391c4d27a10cf751b99bdbd1a1880@apache.org>
Tested-by: Will Berkeley <c74686ab413a71926fc1bc9d16ebfa30e619336a@gmail.com>

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