more wording tweaks

Author: RalfJung

src/behavior-considered-undefined.md

@@ -14,10 +14,14 @@ undefined behavior, it is *unsound*.
 
 <div class="warning">
 
-***Warning:*** The following list is not exhaustive. There is no formal model of
-Rust's semantics for what is and is not allowed in unsafe code, so there may be
-more behavior considered unsafe. We also reserve the right to make some of the
-behavior in that list defined in the future.
+***Warning:*** The following list is not exhaustive; it may grow or shrink.
+There is no formal model of Rust's semantics for what is and is not allowed in
+unsafe code, so there may be more behavior considered unsafe. We also reserve
+the right to make some of the behavior in that list defined in the future. In
+other words, this list does not say that anything will *definitely* always be
+undefined in all future Rust version (but we might make such commitments for
+some list items in the future).
+
 Please read the [Rustonomicon] before writing unsafe code.
 
 </div>