is_reflectable documentation

I've been trying to understand `is_reflectable`, but I fail to get what to pass as second argument. My case looks like this:

```
template
class static_results {
static_assert(pfr::is_pfr_reflectable::value, "RowType must be reflectable");
// Implementation
};
```

I want to check that my user type provided me with a `RowType` that is a structure that can be reflected, i.e. I can call `structure_to_tuple`, `for_each_field` and friends on it.