Revise wording on explicit extern ABI change

Author: traviscross

src/items/external-blocks.md

@@ -110,7 +110,9 @@ r[items.extern.abi.default]
 If the ABI string is not specified, it defaults to `"C"`.
 
 > [!NOTE]
-> It is recommended to always specify the ABI to make it clear which ABI is used. Future versions of Rust may make this a requirement, see [#134986](https://github.com/rust-lang/rust/issues/134986).
+> 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.standard]
 The following ABI strings are supported on all platforms: