Described *1-*3,*e in repl-welcome

I have learned only by chance about the fact that REPL stores evaluation results in *1 - *3 and the last exception in *e. I am certainly not the only one unaware of this so it would be good to include it in the welcome message.

(For random visitors: they are defined in clojure's [`./src/clj/clojure/main.clj`](https://github.com/clojure/clojure/blob/master/src/clj/clojure/main.clj).)