Make clear that status quo ≠ guarantee

Author: petertodd

src/test/run-pass/union/union-nonzero.rs

@@ -6,8 +6,9 @@
 // For example, if a union `U` can contain both a `&T` and a `*const T`, there's definitely no
 // bit-value that an `Option<U>` could reuse as `None`; this test makes sure that isn't done.
 //
-// Secondly, this tests the status quo to not apply such optimizations to types containing unions
-// even if they're theoretically possible. (discussion: https://github.com/rust-lang/rust/issues/36394)
+// Secondly, this tests the status quo (not a guarantee; subject to change!) to not apply such
+// optimizations to types containing unions even if they're theoretically possible. (discussion:
+// https://github.com/rust-lang/rust/issues/36394)
 //
 // Notably this nails down part of the behavior that `MaybeUninit` assumes: that a
 // `Option<MaybeUninit<&u8>>` does not take advantage of non-zero optimization, and thus is a safe