Conditional compilation makes it sound like `#[cfg(foo)]` will evaluate to `true` for `--cfg=foo="bar"` but it doesn't

From the Reference:

A configuration option. The predicate is true if the option is set, and false if it is unset.

And from the Reference

Configuration options are either names or key-value pairs, and are either set or unset.

but rustc's behavior for #[cfg(foo)] is if its set and a name.

A minimal reproduction:

#!/usr/bin/env -S cargo +nightly -Zscript
---
[lints.rust]
unexpected_cfgs = { level = "warn", check-cfg = ['cfg(foo, values(any()))'] }
---

fn main() {
    println!("Hello from the control grup!");
    #[cfg(foo)]
    println!("Hello from `foo`");
    #[cfg(foo = "value")]
    println!("Hello, from `foo=value`");
}

$ RUSTFLAGS='' ./check-cfg-exists.rs  # unset
Hello from the control grup!

$ RUSTFLAGS='--cfg=foo' ./check-cfg-exists.rs  # set with name
Hello from the control grup!
Hello from `foo`

$ RUSTFLAGS='--cfg=foo="value"' ./check-cfg-exists.rs  # set with key-value pair
Hello from the control grup!
Hello, from `foo=value`

Start with the Reference's conditional compilation sections on cfg predicates and option syntax, then run the minimal reproduction with the shown RUSTFLAGS values. Clarify the wording so it accurately describes name and key-value configuration behavior, and verify that the examples and quoted definitions agree with the observed output.