This changes the behavior so that simple aliases that directly alias a
subcommand (with no arguments) pass-through to that subcommand, while
complex aliases (with arguments) show the alias.
So for example, `cargo help b` will show the manpage for `cargo-build`,
while `cargo help my-alias`, aliased to `build --release`, will show
"`my-alias` is aliased to `build --release`".
> Do not merge yet, some options are still undocumented.
Fix#9350 (cargo build -Z help is missing options)
Add a procedural macro to declare `CliUnstable` struct and provide help messages instead of hard-coding help in `src/bin/cargo/cli.rs`
> Flags documentation: https://doc.rust-lang.org/nightly/cargo/reference/unstable.html
Feedback welcome
