docs($locationProvider): mention the default value for hash-prefix ('')

Author: gkalpak

src/ng/location.js

@@ -723,6 +723,7 @@ function $LocationProvider() {
    * @ngdoc method
    * @name $locationProvider#hashPrefix
    * @description
+   * The default value for the prefix is `''`.
    * @param {string=} prefix Prefix for hash part (containing path and search)
    * @returns {*} current value if used as getter or itself (chaining) if used as setter
    */