Document that SegQueue is not completely lock free

SegQueue is not completely lock-free. It effectively contains a spin-lock inside. While this shouldn't affect performance in most cases, it can cause pathological behavior with priority inversion (see this post for details). I am not expert enough to judge if this the right trade-off or not, but I think this warrants a note in the docs

Note that my understanding is very superficial and based on

• https://github.com/crossbeam-rs/crossbeam/pull/279#issuecomment-450490718
• http://www.1024cores.net/home/lock-free-algorithms/queues/bounded-mpmc-queue

and very cursory look at the source code.