RCR 155: Include CONFIG::Config['rubydocdir'] in rbconfig.rb

Ruby would benefit from the introduction of a standard documentation directory. This one seems sensible for Unix-like platforms:

  /usr/[local/]doc/ruby/

A similar path in a Windows install would look like this:

  C:/ruby/doc/ruby

The benefit of agreeing on a standard and implementing it is that package installers could use it to install package documentation in a known place.

In order to implement this, we need to change mkconfig.rb so that in a Ruby installation, the documentation directory is accessible via rbconfig.rb, just as we find the "site_ruby" directory and others in this way.

Therefore, I propose that the following line be included in rbconfig.rb (the first line is an existing one for reference):

  CONFIG["rubylibdir"] = "$(libdir)/ruby/$(ruby_version)"
+ CONFIG["rubydocdir"] = "$(prefix)/doc/ruby"

Roll on decent, installed package documentation!