Document the new Sync Write durability stats

The new Sync Write functionality introduces a bunch of new statistics. Users are interested in knowing about these and should be documented. The stats are defined in Dave Rigby's design spec:

https://docs.google.com/document/d/1DCIzCNXmm3XjkylF19zwNvxcwJFp3bLqSIsQZzTakZ0/edit#heading=h.f47nnbmb4qgh

Most likely these should go along with the rest of the cbstats documentation in:

https://docs.couchbase.com/server/6.5/cli/cbstats-intro.html

Dave Rigby , Daniel Owen FYI.