rustdoc -Z help and -C help are still broken

[jyn514]

I tried this: rustdoc -Zhelp

I expected to see this happen: A list of unstable options

Instead, this happened:

$ rustdoc +stage1 -Zhelp
error: unknown unstable option: `help`

$ rustdoc +stage1 -Chelp
error: unknown codegen option: `help`

The options need to be built after checking for help, not before.
https://github.com/rust-lang/rust/blob/0567674460504d916c254696e1b25082f72a66bf/src/librustdoc/config.rs#L338-L355

cc @GuillaumeGomez

Meta

rustc --version --verbose: 0567674460504d916c254696e1b25082f72a66bf

[GuillaumeGomez]

Thanks for the feedback. I'll send a fix as soon as possible.