(https://github.com/ruby/irb/pull/475)

In the long-term, we want to align with `Pry`, `byebug` and `debug` to
use the `help` command to list all commands, which is what `show_cmds`
currently does. And `show_doc` will be the command to look up Ruby APIs.

By aliasing `show_doc` to the current `help` now, users will have time
to get use to it.