Docs: Document __builtin_nontemporal_load and __builtin_nontemporal_store.

Description

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