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.

src/liballoc/boxed.rs

@@ -382,7 +382,10 @@ impl<T: ?Sized> Box<T> {
     ///
     /// unsafe {
     ///     let ptr = alloc(Layout::new::<i32>()) as *mut i32;
-    ///     *ptr = 5;
+    ///     // In general .write is required to avoid attempting to destruct
+    ///     // the (uninitialized) previous contents of `ptr`, though for this
+    ///     // simple example `*ptr = 5` would have worked as well.
+    ///     ptr.write(5);
     ///     let x = Box::from_raw(ptr);
     /// }
     /// ```