Add documentation for the chrome.sockets API and manifest key.

chrome/common/extensions/docs/templates/articles/manifest/sockets.html

Index: chrome/common/extensions/docs/templates/articles/manifest/sockets.html
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+<h1>sockets</h1>
+
+<p>
+The <code>sockets</code> manifest property declares which permissions are
+available for the $ref:sockets.udp, $ref:sockets.tcp and
+$ref:sockets.tcpServer APIs.
+</p>
+
+<h2 id="manifest">Sample manifest.json</h2>
+<pre data-filename="manifest.json">
+{
+  "name": "My Sockets {{platform}}",
+  "sockets": {
+    "udp": {
+      // Permission for chrome.sockets.udp.send:
+      // The application is allowed to send udp packets
+      // to any host:port combination.
+      "send": "*"
+    },
+    "tcp": {
+      // Permission for chrome.sockets.tcp.connect:
+      // The application is allowed to connect only to the
+      // tcp server on 127.0.0.1 port 80.
+      "connect": "127.0.0.1:80"
+    },
+    "tcpServer": {
+      // Permission for chrome.sockets.tcpServer.listen:
+      // The application is allowed to accept new client
+      // connections on the local address at port 80.
+      "listen": "127.0.0.1:80"
+    }
+  },
+  ...
+}
+</pre>
+
+<section>
+<h2 id="reference">Reference</h2>
+<p class="api_reference">
+{{+partials.type type:apis.manifestTypes.byName.sockets/}}
+</p>
+</section>