Add note about phasing out implicit extern ABI

We're going to stop accepting `extern` without an explicit ABI in the
next edition.  Let's made a note about this.

Author: traviscross

src/items/external-blocks.md

@@ -114,8 +114,12 @@ r[items.extern.abi.rust]
 * `unsafe extern "Rust"` --- The default ABI when you write a normal `fn foo()` in any Rust code.
 
 r[items.extern.abi.c]
-* `unsafe extern "C"` --- This is the same as `extern fn foo()`; whatever the default
-  your C compiler supports.
+* `unsafe extern "C"` --- This is the same as `extern fn foo()`; whatever the default your C compiler supports.
+
+  > [!NOTE]
+  > The `extern` syntax without an explicit ABI is being phased out, so it's better to always write the ABI explicitly.
+  >
+  > For more details, see Rust issue [#134986](https://github.com/rust-lang/rust/issues/134986).
 
 r[items.extern.abi.system]
 * `unsafe extern "system"` --- Usually the same as `extern "C"`, except on Win32, in which case it's `"stdcall"`, or what you should use to link to the Windows API itself.