Document the semantics of atomic_thread_fence operations.

Add atomic_load_<type> and atomic_store_<type>, and explain why they
exist.

Define the synchronizes-with relationship and its effects.

Reorder and revise some of the existing text. For example, more
precisely describe when ordinary accesses are atomic.

Reviewed by: jhb, kib
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D13522