Value Instance does not accept type attribute (despite the documentation's assertion)
[My apologies if this has already been reported -- the obvious search terms turned up more issues than I could check -- maybe I'm just feeble.]

I have a pile of KSY files.  They contain a variety of type definitions, but they all have a few which are common, where "common" means, same name and fields with the same names (but, possibly in a different order or with different sizes, etc.).  In particular, one type definition has a "value" instance, `payload_size`.  However, in some of my KSY files, the return type for the getter function for the value is signed while in others it is unsigned.  I can't use conditional code to access them, and I'd prefer to avoid a typecast in my source (I'm in C++), so, I would like to specify a `type` attribute for the instance, which the the [KSY Reference](http://doc.kaitai.io/ksy_reference.html#spec-instance) documentation suggests I can do:

> Instance specification is very close to [Attribute spec](http://doc.kaitai.io/ksy_reference.html#spec-attribute) (and **_inherits all its properties_**), but it specifies an attribute that lies beyond regular parsing sequence. Typically, each instance is compiled into a lazy reader function/method that will parse (or calculate) requested data on demand, cache the result and return whatever’s been parsed previously on subsequent calls.
> 
> **_Everything that described in [Attribute spec](http://doc.kaitai.io/ksy_reference.html#spec-attribute) can be used_**, except for id, which is useless, because all instances already have name due to map string key.

(Emphasis mine.)

However, the compiler disagrees:
```
/types/record/instances/payload_size/type: invalid key found in value instance, allowed: doc, doc-ref, enum, if, value
```

So...which is one wrong, the compiler or the documentation?  :-)

(While you're pondering that, is there a way that I can use KS's typecasting to work around the problem?  Can I cast the result to a `u2`?  If so, what's the syntax for that?  Thanks!)