Add comment explaining why calling new-truncate is fine

Signed-off-by: Joe Richey <joerichey@google.com>

Author: josephlr

src/structures/idt.rs

@@ -741,6 +741,8 @@ impl<F> Entry<F> {
         let addr = self.pointer_low as u64
             | (self.pointer_middle as u64) << 16
             | (self.pointer_high as u64) << 32;
+        // addr is a valid VirtAddr, as the pointer members are either all zero,
+        // or have been set by set_handler_addr (which takes a VirtAddr).
         VirtAddr::new_truncate(addr)
     }
 }