message: style(typegoose): add note about some top-level types
  re typegoose/typegoose#639
diff: @@ -41,6 +41,7 @@ export * from './typeguards';
  export * as defaultClasses from './defaultClasses';
  export * as errors from './internal/errors';
  export * as types from './types';
  +// the following types are re-exported (instead of just in "types") because they are often used types
  export { DocumentType, Ref, ReturnModelType };
  export { getClassForDocument, getClass, getName } from './internal/utils';
  export { Severity } from './internal/constants';
  