MB-27863: Document SET_WITH_META expiration encoding
MB-27863: Document SET_WITH_META expiration encoding

SET_WITH_META's expiration is always encoded as an absolute number of
seconds since Unix epoch; it does not support "delta" or relative TTLs
from the current time.

Change-Id: Ie8511eed508ce41509ec42b6aa03bd1a6a9a413c
Reviewed-on: http://review.couchbase.org/89330
Reviewed-by: Trond Norbye <60edd2ef23891a753f231b0c6f161dc634079a93@gmail.com>
Tested-by: Dave Rigby <a09264da4832c7ff1d3bf1608a19f4b870f93750@couchbase.com>

daverigby/kv_engine,daverigby/kv_engine,daverigby/kv_engine,daverigby/kv_engine