CCBC-531: Document 64 bit counter on server

Change-Id: I0739573a197223d680774332840a457548638640
Reviewed-on: http://review.couchbase.org/42444
Reviewed-by: Sergey Avseyev <sergey.avseyev@gmail.com>
Tested-by: Mark Nunberg <mnunberg@haskalah.org>