[Extensions Docs] Clarify permission requirements for captureVisibleTab

Add additional details on the permission requirements for the
tabs.captureVisibleTab() method, including that activeTab is required
for sensitive sites that the extension wouldn't otherwise be allowed
access to.

Bug: 839857
Change-Id: I6fa1a68e8dc50bc7c3553fb32740b4c963276395
Reviewed-on: https://chromium-review.googlesource.com/1138634
Commit-Queue: Devlin <rdevlin.cronin@chromium.org>
Reviewed-by: Karan Bhatia <karandeepb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#575519}
1 file changed