=act #13970 Send transistion on goto(CurrentState) in FSM

Squashed and fixed typos in PR prepared by @bambuchaAdm in https://github.com/akka/akka/pull/15360
Thanks Łukasz!

When in `A`:
* `goto(A)` will trigger `onTransition(A -> A)`
* `stay()` will NOT trigger `onTransition`

Includes:
* migration guide
* docs updates
* test