docs($provide): improve examples and explanations

$provide's example seems awkward. Replace with more real-world example,
using an injected service, where the service defined has a good reason to
be a singleton.

There's quite a lot of confusion around $provide:
http://stackoverflow.com/search?q=angularjs+service+vs+factory
Tests for example at: http://jsbin.com/EMabAv/1/edit?js,output