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merge revision(s) 40015: [Backport #8196]

    doumentation by @toolmantim [GH fixes #270]

• timeout.rb: Document Timeout::timeout 0 and nil argument behavior

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/branches/ruby_1_9_3@40055 b2dd03c8-39d4-4d8f-98ff-823fe69b080e