Auto merge of #118213 - Urgau:check-cfg-diagnostics-rustc-cargo, r=petrochenkov

Add more suggestions to unexpected cfg names and values

This pull request adds more suggestion to unexpected cfg names and values diagnostics:
 - it first adds a links to the [rustc unstable book](https://doc.rust-lang.org/nightly/unstable-book/compiler-flags/check-cfg.html) or the [Cargo reference](https://doc.rust-lang.org/nightly/cargo/reference/unstable.html#check-cfg), depending if rustc is invoked by Cargo
 - it secondly adds a suggestion on how to expect the cfg name or value:
    *excluding well known names and values*
    - for Cargo: it suggest using a feature or `cargo:rust-check-cfg` in build script
    - for rustc: it suggest using `--check-cfg` (with the correct invocation)

Those diagnostics improvements are directed towards enabling users to fix the issue if the previous suggestions weren't good enough.

r? `@petrochenkov`

Author: bors