Make position of return type in specs consistent

[michalmuskala]

It appears we have an inconsistency in how we format specs between when they have guards/variables and when they don't when the arguments portion is split over multiple lines.

The case without when is formatted as:

-spec my_fun(
  one :: one_type(),
  two :: two_type()
) -> 
    return_type().

The case with when is formatted as:

-spec my_fun(
  one :: One,
  two :: two_type()
) -> return_type() when
    One :: one_type().

Notice how the return type is in one case allowed to be on the same like as -> and is not allowed in the other, even if it would fit.