merge revision(s) 40440:
update doc.
git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/branches/ruby_2_0_0@40448 b2dd03c8-39d4-4d8f-98ff-823fe69b080e
merge revision(s) 40440:
git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/branches/ruby_2_0_0@40448 b2dd03c8-39d4-4d8f-98ff-823fe69b080e