* lib/yaml.rb, lib/yaml/: [DOC] Document YAML::DBM#key and add
  references to similar methods with more detail. This patch brings
  lib/yaml to 100% documentation coverage.


git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@42529 b2dd03c8-39d4-4d8f-98ff-823fe69b080e