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`

tests/rustdoc-ui/check-cfg/check-cfg.stderr

@@ -4,6 +4,8 @@ warning: unexpected `cfg` condition name: `uniz`
 LL | #[cfg(uniz)]
    |       ^^^^ help: there is a config with a similar name: `unix`
    |
+   = help: to expect this configuration use `--check-cfg=cfg(uniz)`
+   = note: see <https://doc.rust-lang.org/nightly/unstable-book/compiler-flags/check-cfg.html> for more information about checking conditional configuration
    = note: `#[warn(unexpected_cfgs)]` on by default
 
 warning: 1 warning emitted

tests/rustdoc-ui/doctest/check-cfg-test.stderr

@@ -5,6 +5,8 @@ LL | #[cfg(feature = "invalid")]
    |       ^^^^^^^^^^^^^^^^^^^
    |
    = note: expected values for `feature` are: `test`
+   = help: to expect this configuration use `--check-cfg=cfg(feature, values("invalid"))`
+   = note: see <https://doc.rust-lang.org/nightly/unstable-book/compiler-flags/check-cfg.html> for more information about checking conditional configuration
    = note: `#[warn(unexpected_cfgs)]` on by default
 
 warning: 1 warning emitted