"core types include" -> "core types are"

[evanp]

In section 4 "Model", the phrase "core types include" is used, followed by a list of the core types.

Since this list is exhaustive, and there are no other core types, "core types are" may be clearer here.

[trwnh]

Note that we also have #634 which points out that this "core" distinction is confusing if we don't specify any processing considerations for what it means to be a "core" type. There might be better wording here that obsoletes this specific change.

[evanp]

I think it would be a mistake to hang the difference between core types and "extended types" (probably a bad name) off of this particular issue. Let's keep it focused on this include/are minor problem.