Document the activeTab extension permission.

chrome/common/extensions/docs/static/activeTab.html

Index: chrome/common/extensions/docs/static/activeTab.html
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+<div id="pageData-name" class="pageData">The activeTab permission</div>
+
+<p>
+The <code>activeTab</code> permission allows extensions to access the active tab in response to user gestures like clicking an <a href="browserAction.html">extension button</a> or <a href="contextMenus.html">context menu item</a>.
+</p>
+
+<p>
+Many extensions only need access to a page in response to a user gesture on the extension. For example, consider a web clipping extension that has a browser action and context menu item. The extension doesn't need to run automatically on all pages, it only needs to run when one of its UI surfaces is acted upon.
+</p>
+
+<p>
+Without <code>activeTab</code> the extension would need to request both the <code>&lt;all_urls&gt;</code> and <code>tabs</code> permissions, showing a scary warning on installation. With <code>activeTab</code> none of that will be necessary.
+</p>
+
+</p>
+Formally, the <code>activeTab</code> permission allows an extension to use <a href="tabs.html#method-executeScript">tabs.executeScript</a> and <a href="tabs.html#method-insertCSS">tabs.insertCSS</a> on any tab the extension has been invoked on, including any frames with the same security origin. The permission lasts until the tab is navigated.
+</p>
+
+<p>
+The <code>activeTab</code> permission is activated whenever a piece of extension UI is acted upon by a user. At the time of writing, examples of such UI include:
+<ul>
+  <li>clicking on an extension’s browser action,
+  <li>clicking on an extension’s page action,
+  <li>clicking on an extension’s script badge,
+  <li>accepting a suggestion from an extension’s omnibox,
+  <li>invoking an action from an extension’s context menu, and
+  <li>executing a command configured for an extension.
+</ul>
+But this list will grow over time as more types of extension UIs are added.
+</p>