Update Box::from_raw example to generalize better

I know very little about rust, so I saw this example and tried to generalize it by writing,
```
    let layout = Layout::new::<T>();
    let new_obj = unsafe {
        let ptr = alloc(layout) as *mut T;
        *ptr = obj;
        Box::from_raw(ptr)
    };
```
for some more complicated `T`, which ended up crashing with SIGSEGV,
because it tried to `drop_in_place` the previous object in `ptr` which is
of course garbage. I also added a comment that explains why `.write`
is used, but I think adding that comment is optional and may be too verbose
here. I do however think that changing this example is a good idea to
suggest the correct generalization. `.write` is also used in most of the rest
of the documentation here, even if the example is `i32`, so it would additionally
be more consistent.

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