clientv3: clarify Watch close conditions
clientv3: clarify Watch close conditions

The "too slow" comment is rather vague. If the server closes
the watch for being too slow (it doesn't seem to any more), the
watch client should gracefully resume instead of forcing the
user to handle it.

Also removed the 'opts' comment since it wasn't being maintained.

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