Have tidy ensure that we document all unsafe blocks in libcore

src/libcore/ptr/mod.rs

@@ -61,6 +61,8 @@
 //! [`write_volatile`]: ./fn.write_volatile.html
 //! [`NonNull::dangling`]: ./struct.NonNull.html#method.dangling
 
+// ignore-tidy-undocumented-unsafe
+
 #![stable(feature = "rust1", since = "1.0.0")]
 
 use crate::intrinsics;