message: expand explanation of how we treat validity invariants
diff: @@ -23,9 +23,11 @@ Be aware that Miri will not catch all possible errors in your program, and
  cannot run all programs:
  * There are still plenty of open questions around the basic invariants for some
  - types and when these invariants even have to hold, so if you program runs fine
  - in Miri right now that is by no means a guarantee that it is UB-free when
  - these questions get answered.
  + types and when these invariants even have to hold. Miri tries to avoid false
  + positives here, so if you program runs fine in Miri right now that is by no
  + means a guarantee that it is UB-free when these questions get answered. In
  + particular, Miri does currently not check that integers are initialized or
  + that references point to valid data.
  * If the program relies on unspecified details of how data is laid out, it will
  still run fine in Miri -- but might break (including causing UB) on different
  compiler versions or different platforms.
  