Auto merge of #1693 - rust-lang:oli-obk-patch-1, r=RalfJung

Document -Zmiri-compare-exchange-weak-failure-rate

Author: bors

README.md

@@ -195,6 +195,10 @@ 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-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
+  will always fail and `0.0` means it will never fail.
 * `-Zmiri-disable-alignment-check` disables checking pointer alignment, so you
   can focus on other failures, but it means Miri can miss bugs in your program.
   Using this flag is **unsound**.