CCBC-540: Document durability interval as microseconds

Change-Id: Ib01d8efe273b2f8753598c7b5e7a9a7cfc86764a
Reviewed-on: http://review.couchbase.org/42907
Tested-by: Mark Nunberg <mnunberg@haskalah.org>
Reviewed-by: Sergey Avseyev <sergey.avseyev@gmail.com>