stats.org: Improve description of itm_memory stats
stats.org: Improve description of itm_memory stats

Change-Id: I294cad6492026b7ee16cada6fd3dc1a092c69bf2
Reviewed-on: http://review.couchbase.org/70935
Reviewed-by: Daniel Owen <b3fc0e1a51f252986e60be7da121b74d3ac6845f@couchbase.com>
Tested-by: buildbot <80754af91bfb6d1073585b046fe0a474ce868509@couchbase.com>

daverigby/ep-engine,membase/ep-engine,daverigby/ep-engine,couchbase/ep-engine,couchbase/ep-engine,daverigby/kv_engine,couchbase/ep-engine,membase/ep-engine,daverigby/ep-engine,daverigby/kv_engine,membase/ep-engine,daverigby/kv_engine,membase/ep-engine,daverigby/ep-engine,couchbase/ep-engine,daverigby/kv_engine