Docs: Document __builtin_nontemporal_load and __builtin_nontemporal_store.

Summary:
In r247104 I added the builtins for generating non-temporal memory operations,
but now I realized that they lack documentation. This patch adds some.

Differential Revision: http://reviews.llvm.org/D12785

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@247374 91177308-0d34-0410-b5e6-96231b3b80d8