* lib/rdoc.rb, lib/rdoc, test/rdoc: Update to RDoc 4.2.0.alpha(21b241a)

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@47521 b2dd03c8-39d4-4d8f-98ff-823fe69b080e
* 2014-09-11

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@47522 b2dd03c8-39d4-4d8f-98ff-823fe69b080e