Update licenses listed in gem

On top of #921, I'd like to ask if the .gemspec license field can be updated. Currently, it says just Ruby:

https://github.com/ruby/rdoc/blob/d12272b4cf65d8a0004233a047065af093d2345e/rdoc.gemspec#L29

But I think that the field should list all licensed used in the package and therefore it should be:

GPL-2.0 AND Ruby AND BSD-3-Clause AND CC-BY-2.5 and OFL-1.1-RFN

Where the

• GPL-2.0 AND Ruby comes from the LICENSE.rdoc file,
• BSD-3-Clause is specified in lib/rdoc/generator/darkfish.rb
• CC-BY-2.5 should be the silk icon set license
• OFL-1.1-RFN is from lib/rdoc/generator/template/darkfish/css/fonts.css

The LEGAL.rdoc also mentions some test file which follows Perl licensing, however this code is not shipped in resulting packages, so it probably does not qualifies.

BTW the GPL-2.0 is also questionable, because Ruby itself was relicensed quite some time ago, but the RDoc LICENSE file still states GPL-2.0. It might be fine as it is. Not sure.

Start with rdoc.gemspec and compare its license field with LICENSE.rdoc, lib/rdoc/generator/darkfish.rb, lib/rdoc/generator/template/darkfish/css/fonts.css, and LEGAL.rdoc. Verify which licenses apply to files shipped in the gem, resolve the stated GPL-2.0 uncertainty, and update the metadata so it accurately lists the package's licenses.