MB-9477: amended text for Metadata Purge Interval

Change-Id: I8d564e701420fa34b69547ef16bc91bf45dcbba7
Reviewed-on: http://review.couchbase.org/31016
Reviewed-by: Pavel Blagodov <stochmail@gmail.com>
Tested-by: Pavel Blagodov <stochmail@gmail.com>