Ch. 20: tell folks to see the Reference for more ABI info

src/ch20-01-unsafe-rust.md

@@ -333,7 +333,8 @@ Within the `unsafe extern "C"` block, we list the names and signatures of
 external functions from another language we want to call. The `"C"` part defines
 which _application binary interface (ABI)_ the external function uses: the ABI
 defines how to call the function at the assembly level. The `"C"` ABI is the
-most common and follows the C programming language’s ABI.
+most common and follows the C programming language’s ABI. Information about all
+the ABIs Rust supports is available in [the Rust Reference][ABI].
 
 This particular function does not have any memory safety considerations, though.
 In fact, we know that any call to `abs` will always be safe for any `i32`, so we
@@ -481,7 +482,7 @@ _union_. A `union` is similar to a `struct`, but only one declared field is
 used in a particular instance at one time. Unions are primarily used to
 interface with unions in C code. Accessing union fields is unsafe because Rust
 can’t guarantee the type of the data currently being stored in the union
-instance. You can learn more about unions in [the Rust Reference][reference].
+instance. You can learn more about unions in [the Rust Reference][unions].
 
 ### Using Miri to check unsafe code
 
@@ -535,9 +536,10 @@ Whenever you write unsafe code, you can use Miri to help you be more confident
 that the code you have written upholds Rust’s rules.
 
 [dangling-references]: ch04-02-references-and-borrowing.html#dangling-references
+[ABI]: ../reference/items/external-blocks.html#abi
 [differences-between-variables-and-constants]: ch03-01-variables-and-mutability.html#constants
 [extensible-concurrency-with-the-sync-and-send-traits]: ch16-04-extensible-concurrency-sync-and-send.html#extensible-concurrency-with-the-sync-and-send-traits
 [the-slice-type]: ch04-03-slices.html#the-slice-type
-[reference]: ../reference/items/unions.html
+[unions]: ../reference/items/unions.html
 [miri]: https://github.com/rust-lang/miri
 [nightly]: appendix-07-nightly-rust.html