Docs: Using autocomplete for function discovery in IEx

I had to go to the mailing list to discover this trick. Knowing it's autocomplete supplying the information makes it obvious in hindsight, but I had it in my head that it would be part of the h() command. It might also be worth adding a mention of autocomplete to the h(IEx) page, as well, but since this is likely first help page a new user would come across, this may be enough.