Document 'ruby:' to show core documentation in ri

[xstnztk]

ri supports 'ruby:' as a kind of "pseudo gem name" to display files of the core Ruby documentation such as syntax and NEWS. Add the appropriate documentation to the description of options shown by "ri --help".

[xstnztk]

I added this to the ri options description because I was not able to find out how to display the Ruby core documentation in ri. Only when reading the ri source code I stumbled upon the fact that 'ruby' can be used like a gem name to access the core documentation. Maybe this is obvious to other people but I am sure there are some who also do not know this.

English is not my first language, so feel free to adapt the text as you see fit.

[aycabta]

Don't worry, I'm the current maintainer of RDoc but I didn't know 'gem_name:' prefix, I just know now, and I can't speak English well. I think it's probably okay, Grammaly says so too.

@zzak Is there any problem with this patch's English?

[olleolleolle]

This PR looks good! The English reads fine and it explains the ruby: feature well.

[zzak]

@aycabta LGTM