Issue title: Exporting Union Types in Elm Syntax
 Issue body: If we have a submodule with:
`type Animal = Cat | Dog`

we can export Cat and Dog with:
`module Submodule (Animal(..)) where`

and import with
`import Submodule exposing (Animal(..))`

Can we add something that explains this on the [syntax](http://elm-lang.org/docs/syntax) page
 Comments: 
 Comment 0: Isn't there an issue or even a pull request open about this already?
 Comment 1: I guess I was thinking of this: https://github.com/elm-lang/elm-lang.org/pull/512. It's about something else to do with module documentation, though.
 Comment 2: It is mentioned here [Elm Syntax — Modules](http://elm-lang.org/docs/syntax#modules) already, but I agree, it is not explicit. The best I could think of is adding a commentary about the presence of open imports for Union Types in Modules section.
 Comment 3: I think it should be mentioned here too: https://guide.elm-lang.org/types/union_types.html
 Comment 4: Yeah, this is mysterious to a beginner and not emphasized enough in the docs (or at least not enough that I remembered it). This github issue saved me :-)
 Comment 5: Glad I found this github issue, because I couldn't deduct this from [Elm Syntax - Modules](http://elm-lang.org/docs/syntax#modules)
 Comment 6: Puh I am so happy to have seen this issue... Now all my Types go into a single Types.elm
 Comment 7: This was a good 30 minutes of headache for me... glad this issue is the first result when you google it.
 Comment 8: For anyone looking here. Export your union type with `module Module exposing (UnionType(..), otherthing, etc)` and import it with `import Module exposing (UnionType(..))`.
 Comment 9: > For anyone looking here. Export your union type with `module Module exposing (UnionType(..), otherthing, etc)` and import it with `import Module exposing (UnionType(..))`.

I somehow knew that importing it with (..) was possible, but I didnt think of exporting for some reason. ¯\_(ツ)_/¯
 Comment 10: This means that you can't access `Dog` and `Cat` without explicitly exposing them right? Just `import Submodule` isn't enough to access them it seems like.
{'source': 'GithubIssues', 'repo_name_with_owner': 'elm-lang/elm-lang.org', 'issue_id': 131900864, 'repo_stars': 1606.0}