The 'grant_super_admin' action hook triggers any time a users "Super Admin" status is changed.
Super Admin is 'not' an actual role object in WordPress (unlike Subscriber, Contributor, Editor, etc), but a special "user status". As such, in order to give Super Admins new Capabilities you must add the capabilities directly to the users, not the Super Admin role (which does not exist).
A list of WordPress Super Admins is stored in your WordPress install's 'site_admins' site options record.
'grant_super_admin' is location in