Reflect.fields specification regarding class instances

The docs say

```
This method is only guaranteed to work on anonymous structures.
Refer to `Type.getInstanceFields` for a function supporting class instances.
```

However, haxe.Serializer has always used it on class instances in its `serializeFields` function, which makes me think that we can relax this constraint. The only target where it does something else is flash, and I'll be happy to document that accordingly.

This is relevant to #11863 as well.