docs: add warning to wait between FS rebuilds
docs: add warning to wait between FS rebuilds

Customers who are rebuilding multiple Kudu nodes may permanently delete
all healthy replicas for a tablet. This won't prevent data loss in the
event that a only single copy remains and is deleted, but waiting should
help prevent such situations.

Change-Id: I5659c2ba05a0a6b9905213c5df6ba0dcb371f312
Reviewed-on: http://gerrit.cloudera.org:8080/9279
Reviewed-by: Alex Rodoni <b46da68bf9b818cec7052f7cdfe9b3e69485676b@cloudera.com>
Tested-by: Kudu Jenkins
Reviewed-by: Todd Lipcon <2c763d726a8c2f6afda9be7e0e56a2caa6bb5e6b@apache.org>

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