Documentation/upgrades: document Snapshot API error handling
Documentation/upgrades: document Snapshot API error handling

Signed-off-by: Gyuho Lee <3454051ee3341eb69bdf6fca40899cdad4d86496@gmail.com>

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