Rollup merge of #123726 - jieyouxu:command-new-docs, r=Nilstrieb

Clarify `Command::new` behavior for programs with arguments

I mistakenly passed program path along arguments as the same string into `Command::new` a couple of times now. It might be useful to explicitly highlight that `Command::new` intends to accept path to a program, not path to a program plus arguments. Also nudge the user to use `Command::arg` or `Command::args` if they wish to pass arguments.

Author: matthiaskrgr