[PATCH] manual: Document compiler and memory barriers

[Florian Weimer]

* Carlos O'Donell:

> You posted this patch without any rationale or commit message, was
> this an accidental push to the mailing list?

Do we need a rationale for more documentation? 8-)

Parts of <stddef.h> are already covered in the manual, even though the
header is provided by GCC (like <stdatomic.h>).

I want to reference this documentation in them membarrier patch.