lib/tempfile.rb: include doc of Tempfile.open

* lib/tempfile.rb: start rdoc parsing inside singleton class
  definition to include the document there.
  [ruby-core:64157] [Bug #10105]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@47133 b2dd03c8-39d4-4d8f-98ff-823fe69b080e