[ci skip] Added documentation for belongs_to scope parameter

Documented scope parameter for `belongs_to`. I will add documentation to the other associations in associations.rb if people are happy with this PR.
I agree with @al2o3cr that "scope object" doesn't really fit here. Also it feels like you're describing Active Record scopes in general. What do you think about:

> You can pass a second argument as a callable (i.e. proc or lambda) to retrieve a specific record or customize the generated query when you access to the associated object.

Could you also please wrap your additions around 80 chars ?