Hide locked-fullscreen mode from chrome.windows public API docs

This is a private API that can only be used by whitelisted extensions hence taking it off the public
documentation.

Change-Id: Ieb0b31ae371ded18e39e285d445857a0c0d891ae
Reviewed-on: https://chromium-review.googlesource.com/975962
Commit-Queue: Ivan Šandrk <isandrk@chromium.org>
Reviewed-by: Devlin <rdevlin.cronin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#545169}

chrome/common/extensions/api/windows.json

@@ -57,6 +57,7 @@
           "description": "<i>Deprecated since Chrome M59.</i> Docked windows are no longer supported. This state will be converted to \"normal\"."
         }, {
           "name": "locked-fullscreen",
+          "nodoc": true,
           "description": "Locked fullscreen window state. This fullscreen state cannot be exited by user action. It is available only to whitelisted extensions on Chrome OS."
         }]
       },