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.