etcdserver: update NewMember comment to reflect how id is computed
etcdserver: update NewMember comment to reflect how id is computed

Fixes #4573

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