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[Extensions] Add (internal) documention on extension permissions

Extension permissions can be complicated (especially around permission
increases and permission warning generation). Add some documentation
to the codebase for reference.

Change-Id: If938a093439bd9c784bd735091c745ee7194299c
Reviewed-on: https://chromium-review.googlesource.com/1054216
Commit-Queue: Devlin <rdevlin.cronin@chromium.org>
Reviewed-by: Karan Bhatia <karandeepb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#558019}