Add safety note to library/std/src/sys/unix/fd.rs

Co-authored-by: Elichai Turkel <[email protected]>

Author: notriddle

library/std/src/sys/unix/fd.rs

@@ -69,6 +69,7 @@ const fn max_iov() -> usize {
 impl FileDesc {
     pub fn new(fd: c_int) -> FileDesc {
         assert_ne!(fd, -1);
+        // SAFETY: we just asserted that the value is in the valid range and isn't `-1` (the only value bigger than `0xFF_FF_FF_FE` unsigned)
         unsafe { FileDesc { fd } }
     }