message: emphasize that some flags are unsound to use
diff: @@ -166,14 +166,17 @@ up the sysroot. If you are using `miri` (the Miri driver) directly, see the
  Miri adds its own set of `-Z` flags:
  * `-Zmiri-disable-alignment-check` disables checking pointer alignment, so you
  - can focus on other failures.
  + can focus on other failures, but it means Miri can miss bugs in your program.
  + Using this flag is **unsound**.
  * `-Zmiri-disable-stacked-borrows` disables checking the experimental
  [Stacked Borrows] aliasing rules. This can make Miri run faster, but it also
  - means no aliasing violations will be detected.
  + means no aliasing violations will be detected. Using this flag is **unsound**
  + (but the affected soundness rules are experimental).
  * `-Zmiri-disable-validation` disables enforcing validity invariants, which are
  enforced by default. This is mostly useful to focus on other failures (such
  - as out-of-bounds accesses) first. Setting this flag means Miri will miss bugs
  - in your program. However, this can also help to make Miri run faster.
  + as out-of-bounds accesses) first. Setting this flag means Miri can miss bugs
  + in your program. However, this can also help to make Miri run faster. Using
  + this flag is **unsound**.
  * `-Zmiri-disable-isolation` disables host isolation. As a consequence,
  the program has access to host resources such as environment variables, file
  systems, and randomness.
  