Document the -f option to nrepl.cmdline

[xarthna]

While looking at other publicly available deps.edn (Practicalli), I noticed the use of the -f option. I searched for the documentation of this feature and wasn't able to find it. Looking at the source it seems it was skipped in the help output.

I believe the help output should include this feature.

The repl-fn switch discussed:
https://github.com/nrepl/nrepl/blob/master/src/clojure/nrepl/cmdline.clj#L147