admin_index_controller.js: tell users how to list all users

before
![before](https://cloud.githubusercontent.com/assets/56119/4923826/ea7fea92-651d-11e4-9922-b025e6f38f7a.png)

after
![after](https://cloud.githubusercontent.com/assets/56119/4923827/ed3b3052-651d-11e4-9314-32a694cd8856.png)

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