doc: update node migration guide

can we add a description for this example?
This example will walk you through the process to migrate a member in an etcd cluster:

|Name	|Peer URL	|
|-------|-----------|
|infra0	|10.0.1.10:2380|
|infra1	|10.0.1.11	:2380|
|infra2	|10.0.1.12	:2380|

we want to migrate infra1 to infra1' at 10.0.1.13:2380