message: Add docs for `-Zmiri-check-number-validity`
diff: @@ -199,6 +199,10 @@ up the sysroot. If you are using `miri` (the Miri driver) directly, see the
  Miri adds its own set of `-Z` flags, which are usually set via the `MIRIFLAGS`
  environment variable:
  +* `-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
  + enforcing validity invariants. This has no effect when
  + `-Zmiri-disable-validation` is present.
  * `-Zmiri-compare-exchange-weak-failure-rate=<rate>` changes the failure rate of
  `compare_exchange_weak` operations. The default is `0.8` (so 4 out of 5 weak ops will fail).
  You can change it to any value between `0.0` and `1.0`, where `1.0` means it
  