Updated language and concept in extension overview document.

Bug: None
Change-Id: I02409acdeabd8597aa01be164dbcf8f316779f84
Reviewed-on: https://chromium-review.googlesource.com/1070320
Commit-Queue: Crystal Lambert <crystallambert@chromium.org>
Reviewed-by: Devlin <rdevlin.cronin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#565427}