libcore docs should mention that some atomic operations may not be available

[mvirkkunen]

A number of atomic operations such as AtomicUsize::compare_exchange are simply missing on some platforms such as thumbv6m-none-eabi. The methods are behind this condition:

#[cfg(target_has_atomic = "cas")]

It's understandable that these methods aren't available on platforms that don't support compare-and-swap in hardware. However the documentation doesn't mention that at all, which might lead to people assuming that the functionality is available on all platforms. I think the documentation should clearly mention which parts of libcore may be unavailable depending on the platform.