Add documentation of the default values of IRB.conf

When searching the internet have not ever been able to find a page that state the default values for the `IRB.conf` which is usually referenced in `~/.irbrc`

I (again) realized that you can use `puts IRB.conf` however it would be nice to have the output of this within the Ruby docs. This way when searching online you will end up finding the info you are looking for.