Clarify shim implementation comment.

Author: anp

src/librustc_mir/shim.rs

@@ -69,7 +69,8 @@ fn make_shim<'tcx>(tcx: TyCtxt<'tcx>, instance: ty::InstanceDef<'tcx>) -> &'tcx
         // We are generating a call back to our def-id, which the
         // codegen backend knows to turn to an actual virtual call.
         ty::InstanceDef::Virtual(def_id, _) |
-        // ...or we are generating a call to the inner closure defined by #[track_caller]
+        // ...or we are generating a direct call to our #[track_caller] function which
+        // requires an implicit caller location that the virtual call won't pass
         ty::InstanceDef::ReifyShim(def_id) => {
             build_call_shim(
                 tcx,