[Extensions] Remove nodoc from sidePanel.onOpened event.

Expose onOpened event  in documentation.

Reference: w3c/webextensions#779
Bug: 434215968
Change-Id: I157dae60c77e136fa45159ad717a7ee4eeaa01a7
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6893955
Reviewed-by: Devlin Cronin <[email protected]>
Commit-Queue: Oliver Dunk <[email protected]>
Reviewed-by: Oliver Dunk <[email protected]>
Cr-Commit-Position: refs/heads/main@{#1508523}

Author: Harsh Singh

chrome/common/extensions/api/side_panel.idl

@@ -164,6 +164,6 @@ namespace sidePanel {
 
   interface Events {
     // Fired when the extension's side panel is opened.
-    [nodoc] static void onOpened(PanelOpenedInfo info);
+    static void onOpened(PanelOpenedInfo info);
   };
 };