Add helptext to lotus chain export

cli/chain.go

@@ -1030,7 +1030,9 @@ var ChainExportCmd = &cli.Command{
 	ArgsUsage: "[outputPath]",
 	Flags: []cli.Flag{
 		&cli.StringFlag{
-			Name: "tipset",
+			Name:  "tipset",
+			Usage: "specify tipset to start the export from",
+			Value: "@head",
 		},
 		&cli.Int64Flag{
 			Name:  "recent-stateroots",

documentation/en/cli-lotus.md

@@ -2140,7 +2140,7 @@ USAGE:
    lotus chain export [command options] [outputPath]
 
 OPTIONS:
-   --tipset value             
+   --tipset value             specify tipset to start the export from (default: "@head")
    --recent-stateroots value  specify the number of recent state roots to include in the export (default: 0)
    --skip-old-msgs            (default: false)
    --help, -h                 show help (default: false)