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A users guide is added to docs/ for network traffic annotations.
Annotations proto file's comments are updated.

Bug: 656607
Change-Id: I524d0017a9b74c618b6f7088583706eab2bf0fcc
Reviewed-on: https://chromium-review.googlesource.com/753085Reviewed-by: Martin Šrámek <msramek@chromium.org>
Commit-Queue: Ramin Halavati <rhalavati@chromium.org>
Cr-Commit-Position: refs/heads/master@{#515924}