docs($provider): new example, more guidance on factory() vs service()

Currently $provide's example seems awkward. These examples are more real-world
- they use an injected service, and the service defined has a good reason to
be a singleton.

There's quite a lot of confusion around $provide, so I thought it'd
be good to make this page clearer.

Tests for example: http://jsbin.com/EMabAv/1/edit?js,output
Confusion on SO: http://stackoverflow.com/search?q=angularjs+service+vs+factory