fix(doc): add safety notices to Sender::send and Arbiter::access

[Ddystopia]

Hello, those methods are technically unsound, because you can create the future, poll it, it will register the link to the queue, than you can core::mem::forget that future and return from the function, that will deallocate stack frame. But other links will still have those pointers, causing use after free and memory corruption.

[Ddystopia]

Whoops, it's a mistake on my side, that link is not borrowed but owned by the future, thus pin guarantees do apply