merges r20491 from trunk into ruby_1_9_1.
git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/branches/ruby_1_9_1@20508 b2dd03c8-39d4-4d8f-98ff-823fe69b080e
merges r20491 from trunk into ruby_1_9_1.
little documentation fix. a patch from okkez. [ruby-dev:37271]
git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/branches/ruby_1_9_1@20508 b2dd03c8-39d4-4d8f-98ff-823fe69b080e