message: Document -Zmiri-compare-exchange-weak-failure-rate
diff: @@ -251,6 +251,10 @@ environment variable:
  can recognize false positives by "<untagged>" occurring in the message -- this
  indicates a pointer that was cast from an integer, so Miri was unable to track
  this pointer.
  +* `-Zmiri-compare-exchange-weak-failure-rate=<rate>` changes the failure rate of
  + weak atomic 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
  + will always fail and `0.0` means it will never fail.
  Some native rustc `-Z` flags are also very relevant for Miri:
  