docs: Update mut_arg docs on help + version flags

[ClementTsang]

I was recently migrating from clap 3 to 4, and one of the things pointed out as a big change here was that mut_args no longer works for pre-defined help and version flags.

However, in docs.rs, for mut_arg, it still states:

This can be useful for modifying the auto-generated help or version arguments.

Which might be confusing, since if you try that, you'll get something like:

thread 'main' panicked at 'Argument `help` is undefined'

This just removes that line since IMO it'll just lead to confusion with the newer behaviour.