Consider letting help fall back to RI documentation

I noticed that:

irb> help Array
Can't find command `Array`. Please check the command name and try again.

even though IRB can already look up the same documentation with:

irb> show_doc Array

I dug into ShowDoc a bit and saw that it uses RDoc::RI::Driver, so it seems like help could potentially make use of the same functionality.

I'm wondering if it would make sense for help to fall back to RI when the requested name isn't an IRB command.

There seem to be two reasonable ways to handle this:

help Array could tell the user to use show_doc Array.
help Array could just display the RI documentation directly, effectively making help a more general entry point for both IRB commands and Ruby documentation.

The second option seems particularly convenient, but I wasn't sure which behavior would fit better with IRB's intended interface.

Would either of these be desirable? If so, I'd be interested in taking a shot at implementing it.

Start with the IRB help and show_doc entry points, then inspect how ShowDoc uses RDoc::RI::Driver. Resolve whether help should redirect users to show_doc or display RI documentation directly; done means the chosen behavior is implemented and covered by tests.