Update comment on AtomicRMWInst::Nand

As of July 2014, all backends have been updated to implement
AtomicRMWInst::Nand as ~(x & y) (and not as x & ~y, as some did previously).
This was added to the release notes in r212635 (and the LangRef had been
changed), but it seems that we forgot to update the header-file description.

llvm-svn: 218236

llvm/include/llvm/IR/Instructions.h

@@ -651,7 +651,7 @@ public:
     Sub,
     /// *p = old & v
     And,
-    /// *p = ~old & v
+    /// *p = ~(old & v)
     Nand,
     /// *p = old | v
     Or,