Update cache-messages tracking issue.

Author: ehuss

src/doc/src/reference/unstable.md

@@ -270,7 +270,7 @@ private_dep = "2.0.0" # Will be 'private' by default
 ```
 
 ### cache-messages
-* Tracking Issue: TODO
+* Tracking Issue: [#6986](https://github.com/rust-lang/cargo/issues/6986)
 
 The `cache-messages` feature causes Cargo to cache the messages generated by
 the compiler. This is primarily useful if a crate compiles successfully with