Update Box::from_raw example to generalize better
I know very little about rust, so I saw the example here
```
use std::alloc::{alloc, Layout};
unsafe {
let ptr = alloc(Layout:🆕:<i32>()) as *mut i32;
*ptr = 5;
let x = Box::from_raw(ptr);
}
```
and tried to generalize it by writing,
```
let layout = Layout:🆕:<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 think that changing this example to use `.write` instead
would be a good idea to suggest the correct generalization. It is also more
consistent with other documentation items in this file, which use `.write`.
I also added a comment to explain it, but I'm not too attached to that,
and can see it being too verbose in this place.
6556f26991