docs(isNumber): add info about using isFinite to exclude NaN

Closes angular#11230

Author: petebacondarwin

src/Angular.js

@@ -521,6 +521,12 @@ function isString(value) {return typeof value === 'string';}
  * @description
  * Determines if a reference is a `Number`.
  *
+ * This includes the "special" numbers `NaN`, `+Infinity` and `-Infinity`.
+ *
+ * If you wish to exclude these then you can use the native
+ * [`isFinite'](`https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/isFinite)
+ * method.
+ *
  * @param {*} value Reference to check.
  * @returns {boolean} True if `value` is a `Number`.
  */