Copy faillibility warning to DerefMut

Author: jmaargh

library/core/src/ops/deref.rs

@@ -213,6 +213,10 @@ impl<T: ?Sized> Deref for &mut T {
 /// it may be acceptable to panic if the type is misused through programmer
 /// error, for example.
 ///
+/// However, infallibility is not enforced and therefore not guaranteed.
+/// As such, `unsafe` code should not rely on infallibility in general for
+/// soundness.
+///
 /// [book]: ../../book/ch15-02-deref.html
 /// [coercion]: #mutable-deref-coercion
 /// [implementing]: Deref#when-to-implement-deref-or-derefmut