add log level possible values

fix https://github.com/chrislusf/seaweedfs/issues/1989

weed/glog/glog.go

@@ -398,7 +398,7 @@ type flushSyncWriter interface {
 func init() {
 	flag.BoolVar(&logging.toStderr, "logtostderr", false, "log to standard error instead of files")
 	flag.BoolVar(&logging.alsoToStderr, "alsologtostderr", true, "log to standard error as well as files")
-	flag.Var(&logging.verbosity, "v", "log level for V logs")
+	flag.Var(&logging.verbosity, "v", "log levels [0|1|2|3|4], default to 0")
 	flag.Var(&logging.stderrThreshold, "stderrthreshold", "logs at or above this threshold go to stderr")
 	flag.Var(&logging.vmodule, "vmodule", "comma-separated list of pattern=N settings for file-filtered logging")
 	flag.Var(&logging.traceLocation, "log_backtrace_at", "when logging hits line file:N, emit a stack trace")