* error.c: [DOC] Document Exception#cause by @jasonrclark [ci skip]
  [Fixes GH-519] https://github.com/ruby/ruby/pull/519


git-svn-id: svn+ssh://svn.ruby-lang.org/ruby/trunk@44783 b2dd03c8-39d4-4d8f-98ff-823fe69b080e