Added SAFETY comment as request

Author: SkiFire13

library/alloc/src/collections/binary_heap.rs

@@ -495,6 +495,10 @@ impl<T: Ord> BinaryHeap<T> {
         let mut end = self.len();
         while end > 1 {
             end -= 1;
+            // SAFETY: `end` goes from `self.len() - 1` to 1 (both included),
+            //  so it's always a valid index to access.
+            //  It is safe to access index 0 (i.e. `ptr`), because
+            //  1 <= end < self.len(), which means self.len() >= 2.
             unsafe {
                 let ptr = self.data.as_mut_ptr();
                 ptr::swap(ptr, ptr.add(end));