[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}

chrome/common/extensions/api/tabs.json

@@ -746,7 +746,7 @@
       {
         "name": "captureVisibleTab",
         "type": "function",
-        "description": "Captures the visible area of the currently active tab in the specified window. You must have <a href='declare_permissions'>&lt;all_urls&gt;</a> permission to use this method.",
+        "description": "Captures the visible area of the currently active tab in the specified window. In order to call this method, the extension must have either the <a href='declare_permissions'>&lt;all_urls&gt;</a> permission or the <a href='activeTab'>activeTab</a> permission. In addition to sites extensions can normally access, this method allows extensions to capture sensitive sites that are otherwise restricted, including chrome:-scheme pages, other extensions' pages, and data: URLs. These sensitive sites can only be captured with the activeTab permission. File URLs may be captured only if the extension has been granted file access.",
         "parameters": [
           {
             "type": "integer",