dev-guide: document using range_end for prefixes with json
dev-guide: document using range_end for prefixes with json

Lack of a range_end example has caused some confusion.

mickep76/etcd,gyuho/etcd,mkumatag/etcd,rnaveiras/etcd,coreos/etcd,fanminshi/etcd,rnaveiras/etcd,dannysauer/etcd,visheshnp/etcd,whuwxl/etcd,whuwxl/etcd,akauppi/etcd,bluebreezecf/etcd,sebrandon1/etcd,visheshnp/etcd,mkumatag/etcd,coreos/etcd,lorneli/etcd,rnaveiras/etcd,ajityagaty/etcd,YuleiXiao/etcd,cockroachdb/etcd,vimalk78/etcd,ajityagaty/etcd,mqliang/etcd,vimalk78/etcd,sebrandon1/etcd,glevand/coreos--etcd,etcd-io/etcd,sinsharat/etcd,lorneli/etcd,a-robinson/etcd,gyuho/etcd,heyitsanthony/etcd,dannysauer/etcd,mqliang/etcd,li-ang/etcd,joshix/etcd,YuleiXiao/etcd,fanminshi/etcd,akauppi/etcd,etcd-io/etcd,sinsharat/etcd,mqliang/etcd,etcd-io/etcd,xiang90/etcd,cockroachdb/etcd,bdarnell/etcd,mkumatag/etcd,xiang90/etcd,li-ang/etcd,a-robinson/etcd,joshix/etcd,bdarnell/etcd,heyitsanthony/etcd,bluebreezecf/etcd,ajityagaty/etcd,glevand/coreos--etcd,mickep76/etcd