Rollup merge of #117563 - 0xalpharush:docs/into-raw, r=workingjubilee

docs: clarify explicitly freeing heap allocated memory

The documentation for `Box::into_raw` didn't mention `drop` and wondered if I was doing something wrong. Based off [this](https://stackoverflow.com/questions/75441199/rust-how-do-i-correctly-free-heap-allocated-memory), I think it's helpful to include the more concise yet explicit way to free heap allocated memory. This is my first rust PR and I went through https://std-dev-guide.rust-lang.org/development/, but let me know if I missed something :)

library/alloc/src/boxed.rs

@@ -1038,10 +1038,18 @@ impl<T: ?Sized, A: Allocator> Box<T, A> {
     /// use std::ptr;
     ///
     /// let x = Box::new(String::from("Hello"));
-    /// let p = Box::into_raw(x);
+    /// let ptr = Box::into_raw(x);
     /// unsafe {
-    ///     ptr::drop_in_place(p);
-    ///     dealloc(p as *mut u8, Layout::new::<String>());
+    ///     ptr::drop_in_place(ptr);
+    ///     dealloc(ptr as *mut u8, Layout::new::<String>());
+    /// }
+    /// ```
+    /// Note: This is equivalent to the following:
+    /// ```
+    /// let x = Box::new(String::from("Hello"));
+    /// let ptr = Box::into_raw(x);
+    /// unsafe {
+    ///     drop(Box::from_raw(ptr));
     /// }
     /// ```
     ///