* Fixed and clarified comments and parameter names in index.go
Fixed the comment on IndexFunc to say that it returns multiple indexed values.
Clarified the comments and parameter names in the Indexer interface to
consistently use a dichotomy between "storage keys" and "indexed
values".
* Updated comments in index.go based on review by Liggitt
Kubernetes-commit: 4160909495626262ae514bab30fe81a3ff869d39