Update `unexpected_cfgs` lint for Cargo new `check-cfg` config
This PR updates the diagnostics output of the `unexpected_cfgs` lint for Cargo new `check-cfg` config.
It's a simple and cost-less alternative to the build-script `cargo::rustc-check-cfg` instruction.
```toml
[lints.rust]
unexpected_cfgs = { level = "warn", check-cfg = ['cfg(foo, values("bar"))'] }
```
This PR also adds a Cargo specific section regarding check-cfg and Cargo inside rustc's book (motivation is described inside the file, but mainly check-cfg is a rustc feature not a Cargo one, Cargo only enabled the feature, it does not own it; T-cargo even considers the `check-cfg` lint config to be an implementation detail).
This PR also updates the links to refer to that sub-page when using Cargo from rustc.
As well as updating the lint doc to refer to the check-cfg docs.
~**Not to be merged before https://github.com/rust-lang/cargo/pull/13913 reaches master!**~ (EDIT: merged in https://github.com/rust-lang/rust/pull/125237)
`@rustbot` label +F-check-cfg
r? `@fmease` *(feel free to roll)*
Fixes https://github.com/rust-lang/rust/issues/124800
cc `@epage` `@weihanglo`
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