Added some clarifying documentation on registering Mojo interfaces Change-Id: Ie3b80db7f3b8f1048002f76d5ada29bce74e193d Reviewed-on: https://chromium-review.googlesource.com/790041 Reviewed-by: Ken Rockot <[email protected]> Commit-Queue: Sasha Morrissey <[email protected]> Cr-Commit-Position: refs/heads/master@{#522312} CrOS-Libchrome-Original-Commit: 995adc6e6c38fda28e9f1f22b45a99fa0aa7c530