std: Improve docs for atomics. Fix two bugs

This adds lots of docs to the atomics module. Two of the examples
are using the future atomics API and are ignored temporarily.
I discovered a bug in the way AtomicBool's fetch_nand method is
implemented and fixed it by using the correct value for `true`.
I also fixed the implementation of AcqRel fences (it was only doing
a release barrier), and made a "relaxed" fence a failure.
auto merge of #12954 : brson/rust/atomicdocs, r=alexcrichton

This adds lots of docs to the atomics module. Two of the examples
are using the future atomics API (relying on `Share`) and are ignored temporarily.
I discovered a bug in the way AtomicBool's fetch_nand method is
implemented and fixed it by using the correct value for `true`.
I also fixed the implementation of AcqRel fences (it was only doing
a release barrier), and made a "relaxed" fence a failure.
Docify std::vec_ng

I also removed a couple of methods that were silly and added sort.
auto merge of #12955 : sfackler/rust/vec_ng-doc, r=alexcrichton

I also removed a couple of methods that were silly and added sort.