Clarify usage of lcb_get_node constants

Change-Id: Ie01bf9e110c26ec43b6826f6a646e13c743ef20e
Reviewed-on: http://review.couchbase.org/40937
Reviewed-by: Brent Woodruff <brent@couchbase.com>
Tested-by: Mark Nunberg <mnunberg@haskalah.org>
Reviewed-by: Brett Lawson <brett19@gmail.com>