Help for proxy programs should somehow grant a peek behind The Matrix

When rustup acts as a proxy (to cargo, rustc, any others?), it does too good a job of hiding itself. cargo --help gives no hint to noobs like me that there is somewhere else to look at (like rustup --help) to understand examples featuring a cargo +toolchain command.

cargo --help, or at least cargo +stable --help, should either:

• Say up front or down below that they're a wrapper for cargo with an additional option.
• List the +toolchain option among cargo's options (but still make clear that is not part of cargo, not documented there).
• Pass a secret option to the proxy, and/or rely on cargo and others to document the option itself? I sure don't like it.
• Do something else?

Looking at the code, this looks like a non-trivial job for the clap crate to be exploited at src/cli/proxy_mode.rs. Which I'll look into myself, unless someone says it's a bad idea or knows how already.

Start in src/cli/proxy_mode.rs and inspect how rustup forwards help requests for proxy programs such as cargo and rustc. Compare cargo --help and cargo +stable --help, then determine how the proxy relationship and +toolchain option should be exposed without misrepresenting cargo's own documentation. Done means the relevant help output explains where to find rustup's proxy documentation and includes appropriate coverage for the changed behavior.