docs: clarify explicitly freeing heap allocated memory

Author: 0xalpharush

library/alloc/src/boxed.rs

@@ -1044,7 +1044,12 @@ impl<T: ?Sized, A: Allocator> Box<T, A> {
     ///     dealloc(p as *mut u8, Layout::new::<String>());
     /// }
     /// ```
-    ///
+    /// Note: This is equivalent to the following:
+    /// ```
+    /// let x = Box::new(String::from("Hello"));
+    /// let p = Box::into_raw(x);
+    /// drop(Box::from_raw(ptr));
+    /// ```
     /// [memory layout]: self#memory-layout
     #[stable(feature = "box_raw", since = "1.4.0")]
     #[inline]