* lib/irb.rb, lib/irb/ext/save-history.rb: Add documentation on how to
  enabled irb history [ruby-core:51347] [Bug #7679]


git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@39049 b2dd03c8-39d4-4d8f-98ff-823fe69b080e