docs(jsdoc): remove `@kind function` for providers

As @petebacondarwin wrote in #7425, `@kind function` "is only relevant where we want to be able to tell the difference between a service that is an object and a service that is also callable". Also its presence leads to generation of strange 'Usage' sections in the docs for providers.

![image](https://cloud.githubusercontent.com/assets/94334/4076121/5272fd12-2eb7-11e4-897b-9b8f11adb738.png)