Add stat information to doc/stats.org
Add stat information to doc/stats.org

Context:
- vbucket_persist_low_tasks
- defragmenter_tasks
- workload_monitor_tasks

Change-Id: I6365cf2533af0818bb870766545f29aa0d9994a5
Reviewed-on: http://review.couchbase.org/54795
Reviewed-by: Chiyoung Seo <fcffc23db27b19ee4ebba46695ee6b50e09216a2@couchbase.com>
Tested-by: abhinav dangeti <d73ef2279de6577f7bf081737ee47554bbb7dc82@couchbase.com>

daverigby/ep-engine,hisundar/ep-engine,couchbase/ep-engine,membase/ep-engine,daverigby/kv_engine,daverigby/kv_engine,daverigby/ep-engine,daverigby/ep-engine,teligent-ru/ep-engine,teligent-ru/ep-engine,daverigby/kv_engine,hisundar/ep-engine,membase/ep-engine,couchbase/ep-engine,owendCB/ep-engine,owendCB/ep-engine,daverigby/ep-engine,membase/ep-engine,teligent-ru/ep-engine,hisundar/ep-engine,daverigby/kv_engine,membase/ep-engine,couchbase/ep-engine,hisundar/ep-engine,owendCB/ep-engine,owendCB/ep-engine,teligent-ru/ep-engine,couchbase/ep-engine