Doc change in read/write/total accounting metrics

Doc the change wrought by the parent issue. Get it up into the refguide as part of the difference between old hbases and hbase2.

The change confused me and took me a while to untangle.

The read count is for reads that return a non-empty result now. In old hbase1, we'd increment the read-count even if an empty result. This makes reads look bad in YCSB runs when compared to hbase1 (see how totalRequestCount in hbase2 can be way above the sum of reads+writes; it is because it increments even if the row is not found).

Let me get this into refguide otherwise poor old operators will be baffled. The release note on the parent is great; it just needs to be in our guide.