Mojo JS bindings: add documentation.

TBR=rockot@chromium.org,jam@chromium.org
BUG=699569

Change-Id: I5542a46a2f377ac9c6e64b1519f94659ab7fd4bc
Reviewed-on: https://chromium-review.googlesource.com/521789
Reviewed-by: Ken Rockot <rockot@chromium.org>
Reviewed-by: Yuzhu Shen <yzshen@chromium.org>
Commit-Queue: Yuzhu Shen <yzshen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#476719}
4 files changed