Reference extrema from documentation of minmax

I have forgotten a few times what the `extrema` function is called, and end up looking at the documentation for `minmax`. This adds the helpful hint that I would have appreciated.
Merge pull request #6603 from ivarne/patch-1

Reference extrema from documentation of minmax
Doc: cross-reference extrema in minmax
Raise SIGINT on Ctrl-C in REPL if under debugger