Doc the benefits of Send for RcBox

[jumpnbrownweasel]

I am using RcBox to pool Rc allocations under an Arc<Mutex>, move them into worker threads when needed and convert them from RcBox to Rc. When the worker is done, the Rc can be converted back to RcBox and returned to the pool. I think it's worth documenting the benefits of this and pointing out up front that RcBox implements Send (and Sync).