Add some comments to the new queries

src/librustc/query/mod.rs

@@ -55,20 +55,33 @@ rustc_queries! {
             desc { "get the crate HIR" }
         }
 
+        // The indexed HIR. This can be conveniently accessed by `tcx.hir()`.
+        // Avoid calling this query directly.
         query index_hir(_: CrateNum) -> &'tcx map::IndexedHir<'tcx> {
             eval_always
             no_hash
             desc { "index HIR" }
         }
 
+        // The items in a module.
+        // This can be conveniently accessed by `tcx.hir().visit_item_likes_in_module`.
+        // Avoid calling this query directly.
         query hir_module_items(key: DefId) -> &'tcx hir::ModuleItems {
             eval_always
         }
 
+        // An HIR item with a `DefId` that can own other HIR items which do not themselves have
+        // a `DefId`.
+        // This can be conveniently accessed by methods on `tcx.hir()`.
+        // Avoid calling this query directly.
         query hir_owner(key: DefId) -> &'tcx HirOwner<'tcx> {
             eval_always
         }
 
+        // The HIR items which do not themselves have a `DefId` and are owned by another HIR item
+        // with a `DefId`.
+        // This can be conveniently accessed by methods on `tcx.hir()`.
+        // Avoid calling this query directly.
         query hir_owner_items(key: DefId) -> &'tcx HirOwnerItems<'tcx> {
             eval_always
         }