flatten is not an Arg attribute in derive API

[emersonford]

Please complete the following tasks

• I have searched the discussions
• I have searched the open and rejected issues

Rust Version

1.65.0

Clap Version

4.0.26

Minimal reproducible code

use clap::Args;

#[derive(Args)]
struct CommonArgs {
    #[arg(long)]
    pub foo: u32
}

#[derive(Args)]
struct SomeArgs {
    pub bar: u32,
    
    #[arg(flatten)]
    pub common: CommonArgs,
}

Steps to reproduce the bug with the above code

build; playground link

Actual Behaviour

fails to compile with

error: Expected `command` attribute instead of `arg`
  --> src/lib.rs:13:7
   |
13 |     #[arg(flatten)]
   |       ^^^

error: could not compile `playground` due to previous error

Expected Behaviour

should compile, the derive reference (see here) says that flatten is a valid Arg attribute.

Additional Context

workaround is to use #[command(flatten)] or #[clap(flatten)] instead of #[arg(flatten)]. the former feels... incorrect? and iirc the latter is no longer recommended in favor of #[arg(...)].

Debug Output

No response

[epage]

Thanks for catching this! The documentation is updated in master.

#[command(flatten)] is the correct route to go as you are merging together information about the command, rather than setting attributes related to a specific Arg