Refer to the well known names from Cargo check-cfg specifics doc

Author: Urgau

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

@@ -13,6 +13,8 @@ the `unexpected_cfgs` lint and `--check-cfg` flag. It is not intended to provide
 individual details, for that refer to the [`--check-cfg` documentation](../check-cfg.md) and
 to the [Cargo book](../../cargo/index.html).
 
+> The full list of well known cfgs (aka builtins) can be found under [Checking conditional configurations / Well known names and values](../check-cfg.md#well-known-names-and-values).
+
 ## Cargo feature
 
 *See the [`[features]` section in the Cargo book][cargo-features] for more details.*