Document check_rustc_must_implement_one_of boundary in check_attr

Author: darkraider01

compiler/rustc_passes/src/check_attr.rs

@@ -477,6 +477,11 @@ impl<'tcx> CheckAttrVisitor<'tcx> {
         let def_id = hir_id.owner.def_id;
 
         let items = self.tcx.associated_items(def_id);
+        // Syntactic argument parsing and duplicate-name validation are handled by
+        // `RustcMustImplementOneOfParser` in `rustc_attr_parsing`. The checks below remain
+        // in `check_attr` because looking up associated items, checking their kind,
+        // and checking default implementations require `TyCtxt`.
+        //
         // Check that all arguments of `#[rustc_must_implement_one_of]` reference
         // functions in the trait with default implementations
         for ident in list {