MB-8751: tombstone purging text for What's this

Change-Id: I0189f7f67b07f89861950b59ddb6de28706c72e4
Reviewed-on: http://review.couchbase.org/28347
Reviewed-by: Pavel Blagodov <stochmail@gmail.com>
Tested-by: Pavel Blagodov <stochmail@gmail.com>