update docs of missing_abi lint

Author: kadiwa4

compiler/rustc_lint_defs/src/builtin.rs

@@ -3706,7 +3706,7 @@ declare_lint_pass!(UnusedDocComment => [UNUSED_DOC_COMMENTS]);
 
 declare_lint!{
 /// The `missing_abi` lint detects cases where the ABI is omitted from
-/// extern declarations.
+/// `extern` declarations.
 ///
 /// ### Example
 ///
@@ -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
 pubMISSING_ABI,
 Allow,
 "No declared ABI for extern declaration"