Function render_help_for 

pub fn render_help_for<S: AsRef<str>>(root: &CommandSpec, argv: &[S]) -> String

Render --help for the deepest subcommand named by the leading words of argv, so app group leaf --help shows the leaf’s options rather than the group overview. Stops descending at the first flag (or unknown word) and renders whatever level was reached (the root when argv names none).