message: nydus-image/inspect: add help message
diff: @@ -231,7 +231,7 @@ impl RafsInspector {
  Index: {index}
  Name: {name:?}
  Size: {size}
  - Mode: {mode}
  + Mode: 0x{mode:X}
  Nlink: {nlink}
  UID: {uid}
  GID: {gid}
  @@ -377,7 +377,7 @@ impl Executor {
  ("blobs", None) => inspector.cmd_list_blobs(),
  ("prefetch", None) => inspector.cmd_list_prefetch(),
  _ => {
  - println!("Unsupported command or argument is needed!");
  + println!("Unsupported command!");
  {
  Self::usage().unwrap();
  return Err(ExecuteError::IllegalCommand);
  @@ -390,6 +390,17 @@ impl Executor {
  }
  pub(crate) fn usage() -> Result<Option<Value>> {
  + println!(
  + r#"
  + stats: Display global rafs metadata
  + ls: Show files in current directory
  + cd DIR: Change current directory
  + stat FILE_NAME: Show particular information of rafs inode
  + blobs: Show blobs table
  + prefetch: Show prefetch table
  + "#
  + );
  +
  Ok(None)
  }
  }
  