docs: Default lockmode is LOCKMODE_EXC, not LOCKMODE_WAIT

Change-Id: I27cd2838bc5e5cd363a5a5400617987b774d9cc0
Reviewed-on: http://review.couchbase.org/34186
Reviewed-by: Mark Nunberg <mnunberg@haskalah.org>
Tested-by: Mark Nunberg <mnunberg@haskalah.org>