RefCell needs more explanation: heap or stack?

[liufeimath]

The RefCell chapter gives the reader an impression that RefCell is no different from Box other than the additional support of runtime mutability check. However there's another big difference: Box manages object on the heap while RefCell is just a stack variable wrapper. I think this needs to be clearly stated when introducing RefCell to the reader, otherwise it can cause confusions (see this discussion in the forum).