MB-100 Indicate that _Key is actually used now.

Change-Id: Ifc9368dbc76162681ff3f26413143379ae0075d5
Reviewed-on: http://review.couchbase.org/28410
Tested-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>
Reviewed-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>