Add comment addressing Issue rust-lang#73682

Author: jmaargh

library/core/src/ops/deref.rs

@@ -88,6 +88,10 @@
 /// implicitly. In the majority of uses it should be infallible, though 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]: #deref-coercion