Add a comment about the need of GCX_PTR

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

@@ -199,6 +199,10 @@ impl GcxPtr {
 
 unsafe impl Sync for GcxPtr {}
 
+/// 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.
 scoped_tls::scoped_thread_local! {
  pub static GCX_PTR: GcxPtr
 }