Undocumented Derive Attributes

[Iron-E]

On master, looking at the derive_ref, I have come across several options from other places in the documentation which do not appear to be documented in that file:

1. group (example)
2. requires (example)
3. value_name (example)

I can only find mention of value_parser and required_if_eq on the derive_ref.

If this issue is invalid feel free to close.

[epage]

These are documented by delegating to the builder documentation

Raw attributes: Any Arg method can also be used as an attribute, see Terminology for syntax.

e.g. #[clap(max_values(3))] would translate to arg.max_values(3)

See https://github.com/clap-rs/clap/blob/master/examples/derive_ref/README.md#arg-attributes

[epage]

I've created #4090 to further discuss the documentation of raw attributes as this keeps coming up