* lib/irb.rb, lib/irb/context.rb: Add documentation on how to enable
  auto-indentation and autocompletion using irbrc and irb_context
  [ruby-core:51209] [Bug #7642] and [ruby-core:51348] [Bug #7680]


git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@39048 b2dd03c8-39d4-4d8f-98ff-823fe69b080e