etcdmain: Add max-snapshots and max-wals to help

Based on the configuration doc, seems these two flags are missing
in the help. So add them and the descriptions are from config.go in
the same directory.

Signed-off-by: Yiqiao Pu <ypu@redhat.com>

etcdmain/help.go

@@ -14,6 +14,8 @@
 
 package etcdmain
 
+import "strconv"
+
 var (
 	usageline = `usage: etcd [flags]
        start an etcd server
@@ -43,6 +45,10 @@ member flags:
 		list of URLs to listen on for peer traffic.
 	--listen-client-urls 'http://localhost:2379,http://localhost:4001'
 		list of URLs to listen on for client traffic.
+	--max-snapshots '` + strconv.Itoa(defaultMaxSnapshots) + `'
+		maximum number of snapshot files to retain (0 is unlimited).
+	--max-wals '` + strconv.Itoa(defaultMaxWALs) + `'
+		maximum number of wal files to retain (0 is unlimited).
 	-cors ''
 		comma-separated whitelist of origins for CORS (cross-origin resource sharing).