docs(rustc): Improve discoverable of Cargo docs

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] section 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",

src/doc/rustc/src/check-cfg.md

@@ -11,8 +11,8 @@ development process.
 In order to accomplish that goal, `rustc` accepts the `--check-cfg` flag, which specifies
 whether to check conditions and how to check them.
 
-> **Note:** No implicit expectation is added when using `--cfg`. Users are expected to
-pass all expected names and values using the _check cfg specification_.
+> **Note:** For interacting with this through Cargo,
+see [Cargo Specifics](check-cfg/cargo-specifics.md) page.
 
 [^reachable]: `rustc` promises to at least check reachable `#[cfg]`, and while non-reachable
 `#[cfg]` are not currently checked, they may well be checked in the future without it being a
@@ -23,6 +23,9 @@ breaking change.
 To specify expected names and values, the _check cfg specification_ provides the `cfg(...)`
 option which enables specifying for an expected config name and it's expected values.
 
+> **Note:** No implicit expectation is added when using `--cfg`. Users are expected to
+pass all expected names and values using the _check cfg specification_.
+
 It has this basic form:
 
 ```bash