message: nydusctl: print more paticular help message
  Just give users more hints about how to use this client tool correctly
diff: @@ -37,15 +37,21 @@ async fn main() -> Result<()> {
  .takes_value(false)
  .global(false),
  )
  - .subcommand(SubCommand::with_name("info").help("Get nydusd working status"))
  + .subcommand(SubCommand::with_name("info").about("Get nydusd working status"))
  .subcommand(
  SubCommand::with_name("set")
  - .help("Configure nydusd")
  + .about(
  + "Configure nydusd parameters in format: set KIND VALUE where KIND can be \"log-level\"",
  + )
  + .help(
  + r#"Acceptable Items:
  + log-level: trace, debug, info, warn, error"#,
  + )
  .arg(
  Arg::with_name("KIND")
  .help("what item to configure")
  .required(true)
  - .possible_values(&["log_level"])
  + .possible_values(&["log-level"])
  .takes_value(true)
  .index(1),
  )
  @@ -59,7 +65,9 @@ async fn main() -> Result<()> {
  )
  .subcommand(
  SubCommand::with_name("metrics")
  - .help("Configure nydusd")
  + .about(
  + "Query nydus metrics. Possible metrics category: fsstats; blobcache; backend",
  + )
  .arg(
  Arg::with_name("category")
  .help("Show the category of metrics: blobcache, backend, fsstats")
  