Improve checker.check comment
@@ -17,7 +17,16 @@ local check_field
-- Type-check a Titan module
--
--- Sets a _type field on some nodes. TODO: what nodes?
+-- Sets a _type field on some AST nodes:
+-- - Value declarations:
+-- - ast.Toplevel.Func
+-- - ast.Toplevel.Var
+-- - ast.Decl.Decl
+-- - ast.Exp
+-- - ast.Var
+--
+-- Sets a _field_types field on ast.Toplevel.Record nodes, mapping field names
+-- to their types.
--
-- @ param prog AST for the whole module
-- @ return true or false, followed by as list of compilation errors
