doc: add response header doc into api
doc: add response header doc into api

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