Remove references to _.

isNull.js

@@ -2,17 +2,16 @@
  * Checks if `value` is `null`.
  *
  * @static
- * @memberOf _
  * @since 0.1.0
  * @category Lang
  * @param {*} value The value to check.
  * @returns {boolean} Returns `true` if `value` is `null`, else `false`.
  * @example
  *
- * _.isNull(null);
+ * isNull(null);
  * // => true
  *
- * _.isNull(void 0);
+ * isNull(void 0);
  * // => false
  */
 function isNull(value) {