CCBC-529/CCBC-532: More conflicting option enforcement

This adds enforcement on not passing `exptime` if no `create` option was
set for `lcb_arithmetic`. Additionally, some more documentation was
added to the headers to inform the user of these restrictions.

Change-Id: I93189a270fd3629bdf198f823160de3cb8b11e6f
Reviewed-on: http://review.couchbase.org/42393
Reviewed-by: Sergey Avseyev <sergey.avseyev@gmail.com>
Tested-by: Mark Nunberg <mnunberg@haskalah.org>