etcdctl: add docs for configuration flags and env vars
etcdctl: add docs for configuration flags and env vars

zaphod-concur/etcd,endocode/etcd,wantonsolutions/dinv-etcd,etcd-io/etcd,zaphod-concur/etcd,hongchaodeng/etcd,vimalk78/etcd,li-ang/etcd,yudai/etcd,a-robinson/etcd,lumjjb/etcd,OyTao/etcd_learning,achanda/etcd,gyuho/etcd,mkumatag/etcd,heyitsanthony/etcd,li-ang/etcd,rchicoli/etcd,linuxcer/etcd,xiang90/etcd,ajityagaty/etcd,bluebreezecf/etcd,dnaeon/etcd,lorneli/etcd,a-robinson/etcd,mqliang/etcd,mkumatag/etcd,yudai/etcd,glevand/coreos--etcd,wantonsolutions/dinv-etcd,bdarnell/etcd,cockroachdb/etcd,ravigadde/etcd,tguangch/etcd,handsomesun/etcd,shawnps/etcd,wantonsolutions/dinv-etcd,fanminshi/etcd,sebrandon1/etcd,nekto0n/etcd,jlamillan/etcd,qqsun8819/etcd,vimalk78/etcd,westhood/etcd,armstrongli/etcd,jlamillan/etcd,glycerine/etcd,raoofm/etcd,qqsun8819/etcd,dnaeon/etcd,ktateish/etcd,mkumatag/etcd,sebrandon1/etcd,mqliang/etcd,plasticbox/etcd,rnaveiras/etcd,coreos/etcd,jonboulle/etcd,whuwxl/etcd,purpleidea/etcd,siddontang/etcd,armstrongli/etcd,dannysauer/etcd,nekto0n/etcd,swingbach/etcd,YuleiXiao/etcd,bluebreezecf/etcd,etcd-io/etcd,ajityagaty/etcd,sinsharat/etcd,YuleiXiao/etcd,tamird/etcd,insequent/etcd,handsomesun/etcd,siddontang/etcd,visheshnp/etcd,luxas/etcd,akauppi/etcd,hongchaodeng/etcd,CrazyJvm/etcd,lumjjb/etcd,CrazyJvm/etcd,StorReduce/etcd,purpleidea/etcd,mqliang/etcd,chipironcin/etcd,mickep76/etcd,tamird/etcd,swingbach/etcd,glycerine/etcd,sofuture/etcd,dannysauer/etcd,StorReduce/etcd,jonboulle/etcd,gyuho/etcd,plasticbox/etcd,glevand/coreos--etcd,ravigadde/etcd,joshix/etcd,visheshnp/etcd,sinsharat/etcd,endocode/etcd,chipironcin/etcd,rnaveiras/etcd,OyTao/etcd_learning,luxas/etcd,shawnps/etcd,xiang90/etcd,sofuture/etcd,bdarnell/etcd,rchicoli/etcd,cockroachdb/etcd,heyitsanthony/etcd,joshix/etcd,achanda/etcd,linuxcer/etcd,ktateish/etcd,coreos/etcd,etcd-io/etcd,rnaveiras/etcd,fanminshi/etcd,ajityagaty/etcd,raoofm/etcd,lorneli/etcd,insequent/etcd,westhood/etcd,tguangch/etcd,mickep76/etcd,akauppi/etcd,whuwxl/etcd