docs(rustc): Link unexpected_cfgs to the Cargo.toml docs

This is the first time we have a lint with configuration exposed in
`Cargo.toml`.
When this was done, interest was expressed in using this for other cases
in the future.
To this end, we need to make the documentation for the lint
configuration discoverable from the documentation for that lint.

Author: epage

compiler/rustc_lint_defs/src/builtin.rs

@@ -3256,7 +3256,11 @@ declare_lint! {
     /// See the [Checking Conditional Configurations][check-cfg] section for more
     /// details.
     ///
+    /// See the [Cargo Specifics][unexpected_cfgs_lint_config] for configuring this lint in
+    /// `Cargo.toml`.
+    ///
     /// [check-cfg]: https://doc.rust-lang.org/nightly/rustc/check-cfg.html
+    /// [unexpected_cfgs_lint_config]: https://doc.rust-lang.org/nightly/rustc/check-cfg/cargo-specifics.html#check-cfg-in-lintsrust-table
     pub UNEXPECTED_CFGS,
     Warn,
     "detects unexpected names and values in `#[cfg]` conditions",