Formatting specs with 'when' keyword

[erszcz]

Based on esl/exml#52 it turns out that the formatter doesn't currently handle when clauses in specs well (probably in types, too, though they don't seem to be useful there). The specific points were:

esl/exml#52 (comment)
https://github.com/esl/exml/pull/52/files#r393059362
https://github.com/esl/exml/pull/52/files#r393072500

Given an input spec and function head:

-spec subelement_with_name_and_ns(exml:element(), binary(), binary(), Other) ->
    exml:element() | Other.
subelement_with_name_and_ns(Element, Name, NS, Default) -> ...

The formatter currently prints:

-spec subelement_with_name_and_ns(exml:element(),
                                  binary(),
                                  binary(),
                                  Other) -> exml:element() | Other.

We're proposing the formatter to automatically expand long specs (of more than 3 arguments) to the following form instead:

-spec subelement_with_name_and_ns(Element, Name, NS, Default) -> R when
      Element :: exml:element(),
      Name :: binary(),
      NS :: binary(),
      Default :: Other,
      R :: exml:element() | Other.

Alternatively, it would be beneficial if it could introduce line breaks before the return value, not between the function arguments:

-spec subelement_with_name_and_ns(exml:element(), binary(), binary(), Other) ->
      exml:element() | Other.