Improve extension settings documentation
@artfulrobot has written a good answer here https://civicrm.stackexchange.com/questions/3458/how-do-i-add-a-new-settings-page-for-my-extension/21597#21597 which is better than the current documentation: https://docs.civicrm.org/dev/en/latest/framework/setting/#creating-a-new-setting-in-an-extension