Add tracking issue for --output-format=doctest

Author: GuillaumeGomez

src/doc/rustdoc/src/unstable-features.md

@@ -546,6 +546,8 @@ information.
 
 ### doctest
 
+Tracking issue: [#134529](https://github.com/rust-lang/rust/issues/134529)
+
 `--output-format doctest` emits JSON which gives you information about doctests in the provided crate.
 
 You can use this option like this: