Clarify where the indirect UB due to write-write races comes from

[RalfJung]

Summary

This is based on https://bugs.llvm.org/show_bug.cgi?id=42435#c3.

[efriedma]

LGTM

[RalfJung]

Thanks!

I cannot land commits myself, could you help with that?

[efriedma]

r366855.

llvm/docs/Atomics.rst

for (int i = 0; i < 100; i++) {
       if (a[i])
         xtemp += 1;
     }
     x = xtemp;
   }
 
 However, LLVM is not allowed to transform the former to the latter: it could
 indirectly introduce undefined behavior if another thread can access ``x`` at
-the same time. (This example is particularly of interest because before the
-concurrency model was implemented, LLVM would perform this transformation.)
+the same time. That thread would read `undef` instead of the value it was
+expecting, which can lead to undefined behavior down the line. (This example is
+particularly of interest because before the concurrency model was implemented,
+LLVM would perform this transformation.)
 
 Note that speculative loads are allowed; a load which is part of a race returns
 ``undef``, but does not have undefined behavior.
 
 Atomic instructions
 ===================
 
 For cases where simple loads and stores are not sufficient, LLVM provides