Update API documentation on CombinedError.

[parkerziegler]

Currently our API documentation on CombinedError is non-existent. This is an important part of the urql API and deserves to be covered in more depth. API documentation should include the following:

• Type signature and description of the CombinedError.combinedError record.
• Examples showing how to pattern match combinedError on the networkError and graphQLErrors fields.
• Example showing the message field added in Expose message field on combinedError #105.

In addition, we should add a .rei file for this module as long as we're here.

[Schmavery]

One thing that I found a little funny with this module is that the "type t" is the js version of this type, which afaict isn't really used that much by users. Mostly I would interact with the combinedError type (the record version). Caused some confusion once when trying to annotate something with UrqlCombinedError.t fwiw
Not a big deal but unless I'm missing something, could be worth swapping the name of the user-facing type to be t.

[parkerziegler]

Definitely agreed, that was an oversight on my part @Schmavery. I think initially I was planning to leave the Js.t as the exposed API, but now that the record is what people are actually working with I'm going to adjust the API such that what today is UrqlCombinedError.combinedError becomes UrqlCombinedError.t.