docs(jsdoc): remove `@kind function` from everything but services

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)
Normally we aren't adding new providers, we're adding new methods --- so it makes sense to blindly copy the `@function` tag. It's not clear why it was ever applied to Providers though.