Document what "memory model" guarantees each Atomic method provides

[pitr-ch]

[pitr-ch]

Follow up of ruby-concurrency/atomic#2

[pitr-ch]

It looks like the C code uses a full memory barrier, see https://github.com/ruby-concurrency/concurrent-ruby/blob/master/ext/concurrent/atomic_reference.c#L29-L38

[jdantonio]

@pitr-ch Has this been handled as part of the documentation of the synchronization layer?

[pitr-ch]

it was not. I'll add something quickly.