[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.
Maybe you can remove this example and the previous one as they are almost the same apart from the fact that the last one shows how to pass extra arguments. Also, this seems more common to pass a scope to a `belongs_to` association that uses something like `joins`:

~~~ruby
belongs_to :user, -> { joins(:friends) }
~~~

What do you think ? Thanks for your contribution so far ! :-)