fix(textfield): Add function docs

Author: anton-kachurin

packages/mdc-textfield/foundation.js

@@ -260,6 +260,9 @@ class MDCTextfieldFoundation extends MDCFoundation {
     }
   }
 
+  /**
+   * @private
+   */
   updateDefaultValidity_() {
     if (!this.useCustomValidityChecking_) {
       const input = this.getNativeInput_();
@@ -290,6 +293,7 @@ class MDCTextfieldFoundation extends MDCFoundation {
    * Updates the state of the Textfield's help text based on validity and
    * the Textfield's options.
    * @param {boolean} isValid
+   * @private
    */
   updateHelptext_(isValid) {
     const {HELPTEXT_PERSISTENT, HELPTEXT_VALIDATION_MSG} = MDCTextfieldFoundation.cssClasses;