Add a comment about the need of GCX_PTR

compiler/rustc_middle/src/ty/context/tls.rs

@@ -200,5 +200,9 @@ impl GcxPtr {
 unsafe impl Sync for GcxPtr {}
 
 scoped_tls::scoped_thread_local! {
+    /// This is needed to allow the deadlock handler access to `GlobalCtxt` to look for query cycles.
+    /// It cannot use the `TLV` global is that's only guaranteed to be defined on the thread creating
+    /// the `GlobalCtxt`. Other thread have access to the `TLV` only inside Rayon jobs, but the
+    /// deadlock handler is not called inside such a job.
     pub static GCX_PTR: GcxPtr
 }