MB-7097: document size limit is 2.5 kb

Change-Id: I7b8b6d935467913da0254579a60e144a3824665e
Reviewed-on: http://review.couchbase.org/27458
Reviewed-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>