[Elm] Description conversion to function comment can be improved

Description

Comments describing a function or declaration start with {-| instead of {-. They belong to the next declaration.

{-| Representation of fast immutable arrays. You can create arrays of integers
(`Array Int`) or strings (`Array String`) or any other type of value you can
dream up.
-}
type Array a = Array

https://github.com/elm-lang/core/blob/5.1.1/src/Array.elm#L39-L43

The codegen will generate this (including newlines) instead (the | character is missing):

{-
   My description
-}

and if there is no description at all, the output will be a comment with no content. Instead the comment should be omitted.

{-

-}

Swagger-codegen version

2.3.1

Suggest a fix/enhancement

The parentheses are hardcoded. There could be more occurrences.

https://github.com/swagger-api/swagger-codegen/blob/d6539ba3c83c081e95157433f2e1a7639d14dc6d/modules/swagger-codegen/src/main/resources/elm/model.mustache#L14-L16

https://github.com/swagger-api/swagger-codegen/blob/d6539ba3c83c081e95157433f2e1a7639d14dc6d/modules/swagger-codegen/src/main/resources/elm/api.mustache#L16-L18

Start with modules/swagger-codegen/src/main/resources/elm/model.mustache and elm/api.mustache, then compare their description output with the Elm example in the issue. Update the templates so descriptions use the function-comment marker and empty descriptions produce no comment; check both generated outputs for the unwanted blank comment.