* array.c: [DOC] Add note about negative indices in Array overview
  By @ckaenzig [Fixes GH-427] https://github.com/ruby/ruby/pull/427


git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@43570 b2dd03c8-39d4-4d8f-98ff-823fe69b080e