Add a useful comment to NodeId::hash_stable.

Author: nnethercote

compiler/rustc_ast/src/node_id.rs

@@ -22,6 +22,8 @@ rustc_index::newtype_index! {
 impl HashStable for NodeId {
     #[inline]
     fn hash_stable<Hcx: HashStableContext>(&self, _: &mut Hcx, _: &mut StableHasher) {
+        // This impl is never called but is necessary for types implementing `HashStable` such as
+        // `MainDefinition` and `DocLinkResMap` (both of which occur in `ResolverGlobalCtxt`).
         panic!("Node IDs should not appear in incremental state");
     }
 }