doc: add doc for proxy
doc: add fallback to discovery section in clustering.md
Merge pull request #1671 from xiang90/proxy_doc

doc: add doc for proxy
etcdserver: ensure initial-advertise-peer-urls match initial-cluster

This adds a check to setupCluster to ensure that the list of URLs
specified in `initial-advertise-peer-urls` matches those configured in
`initial-cluster` for this node. Also updates the documentation to
clarify this and address some changes in wording.
etcdserver: move peer URLs check to config