Update unexpected_cfgs lint for Cargo new check-cfg config

[Urgau]

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.

[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 rust-lang/cargo#13913 reaches master! (EDIT: merged in #125237)

@rustbot label +F-check-cfg
r? @fmease (feel free to roll)
Fixes #124800
cc @epage @weihanglo