*: make it clear that proxy only supports v2 api now

Documentation/op-guide/configuration.md

@@ -121,7 +121,7 @@ To start etcd automatically using custom settings at startup in Linux, using a [
 + env variable: ETCD_DISCOVERY_SRV
 
 ### --discovery-fallback
-+ Expected behavior ("exit" or "proxy") when discovery services fails.
++ Expected behavior ("exit" or "proxy") when discovery services fails. "proxy" supports v2 API only.
 + default: "proxy"
 + env variable: ETCD_DISCOVERY_FALLBACK
 
@@ -142,7 +142,7 @@ To start etcd automatically using custom settings at startup in Linux, using a [
 
 ## Proxy Flags
 
-`--proxy` prefix flags configures etcd to run in [proxy mode][proxy].
+`--proxy` prefix flags configures etcd to run in [proxy mode][proxy]. "proxy" supports v2 API only.
 
 ### --proxy
 + Proxy mode setting ("off", "readonly" or "on").

etcdmain/etcd.go

@@ -400,6 +400,8 @@ func startEtcd(cfg *config) (<-chan struct{}, error) {
 
 // startProxy launches an HTTP proxy for client communication which proxies to other etcd nodes.
 func startProxy(cfg *config) error {
+	plog.Notice("proxy: this proxy supports v2 API only!")
+
 	pt, err := transport.NewTimeoutTransport(cfg.peerTLSInfo, time.Duration(cfg.ProxyDialTimeoutMs)*time.Millisecond, time.Duration(cfg.ProxyReadTimeoutMs)*time.Millisecond, time.Duration(cfg.ProxyWriteTimeoutMs)*time.Millisecond)
 	if err != nil {
 		return err

etcdmain/help.go

@@ -75,6 +75,7 @@ clustering flags:
 		discovery URL used to bootstrap the cluster.
 	--discovery-fallback 'proxy'
 		expected behavior ('exit' or 'proxy') when discovery services fails.
+		"proxy" supports v2 API only.
 	--discovery-proxy ''
 		HTTP proxy to use for traffic to discovery service.
 	--discovery-srv ''
@@ -85,6 +86,7 @@ clustering flags:
 		auto compaction retention in hour. 0 means disable auto compaction.
 
 proxy flags:
+	"proxy" supports v2 API only.
 
 	--proxy 'off'
 		proxy mode setting ('off', 'readonly' or 'on').