message: explain what we mean by 'unsound'
diff: @@ -236,7 +236,8 @@ up the sysroot. If you are using `miri` (the Miri driver) directly, see the
  [miri-flags]: #miri--z-flags-and-environment-variables
  Miri adds its own set of `-Z` flags, which are usually set via the `MIRIFLAGS`
  -environment variable:
  +environment variable. Some of these are **unsound**, which means they can lead
  +to Miri failing to detect cases of undefined behavior in a program.
  * `-Zmiri-check-number-validity` enables checking of integer and float validity
  (e.g., they must be initialized and not carry pointer provenance) as part of
  