docs: document Runtime

[Ivshti]

the current implementation of Runtime has two gotchas that need to be addressed in the documentation:

• .dispatch returns a future; if not handled, it will not run; worse yet, in JS it will cause a crash cause the future is dropped but the underlying promise lives on
• receiver never closes; not even all senders are dropped (?? it should close when the instance of runtime is dropped, but check that; if so, then this can be discarded)