The Update Widget Connector Macro Documentation says that

Add this macro to your page

To add the Widget Connector macro to a page:

• From the editor toolbar, choose Insert > Other Macros.
• Choose Widget Connector from the Media category.
• Enter the URL you want to display.
• Choose Insert.
• You can then publish your page to see the macro in action

For Confluence 7.5/7.6 Widget Connector macro is available under External Content category instead of Media category hence the request is to update the documentation accordingly