paper-input documentation lacks details on field validation #888

The documentation for paper-input includes styles for .invalid class, as well as the error string attribute which bears this description: "The message to display if the input value fails validation. If this is unset or the empty string, a default message is displayed depending on the type of validation error."

But there is no explanation of how to apply validation errors, what "types" of validation errors exist or which specific types would trigger the default validation message.