Here is the link
http://docs.mongodb.org/manual/core/index-multikey/#index-arrays-with-embedded-documents.
Index arrays with embedded documents are a non-trivial topic. The example provided on the doc page is too simplistic; it contains only one embedded document attribute. A more complex example may better demonstrate the full functionality of the feature. For example, include 2 or 3 embedded fields instead of 1. Also, it would be more informative to have a sorting example and show different sort order by different indexes.