update docs of missing_abi lint

Author: kadiwa4

compiler/rustc_lint_defs/src/builtin.rs

@@ -3720,10 +3720,12 @@ declare_lint! {
     ///
     /// ### Explanation
     ///
-    /// Historically, Rust implicitly selected C as the ABI for extern
-    /// declarations. We expect to add new ABIs, like `C-unwind`, in the future,
-    /// though this has not yet happened, and especially with their addition
-    /// seeing the ABI easily will make code review easier.
+    /// For historic reasons, Rust implicitly selects C as the default ABI for
+    /// extern declarations. [Other ABIs] like `C-unwind` and `system` have been
+    /// added since then, and especially with their addition seeing the ABI
+    /// easily makes code review easier.
+    ///
+    /// [Other ABIs]: https://doc.rust-lang.org/reference/items/external-blocks.html#abi
     pub MISSING_ABI,
     Allow,
     "No declared ABI for extern declaration"