Improve documentation of UnknownArrayHandle::IsType

Under some circumstances, `IsType` can return false for a type that
exactly matches the type used to create the `UnknownArrayHandle`.
This is because types like `ArrayHandleCast` get converted to
the array they contain. Thus, `IsType` will return false when it
is expected to be true. In most cases, you should use
`CanConvert`. The `IsType` documentation now points you there.
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