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 $provide:
http://stackoverflow.com/search?q=angularjs+service+vs+factory
Tests for example at: http://jsbin.com/EMabAv/1/edit?js,output
docs($provide): improve docs and examples further

Improve the "tracking" service example by adding a configuration option.
Get better formatting of the generated code samples using <pre> tags.
Move the detailed explanations into each function's documentation block.
Improve the overview and list the constituent functions by significance.

Closes #4302