dir.c: Dir.exists? is deprecated

* dir.c (rb_dir_exists_p): [DOC] Document that Dir.exists? is
  deprecated.  [ruby-core:64135] [Bug #10102]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@47134 b2dd03c8-39d4-4d8f-98ff-823fe69b080e