node.html.twig documentation has been simplified

node.html.twig previously documented many variables, including:

 * - readmore: Flag for more state. Will be true if the teaser content of the
 *   node cannot hold the main body content.
 * - logged_in: Flag for authenticated user status. Will be true when the
 *   current user is a logged-in member.
 * - is_admin: Flag for admin user status. Will be true when the current user
 *   is an administrator.

readmore is invalid and has not worked for several years.

logged_in and is_admin are available in all Twig templates so there is no need to document these individually on every template.

These have been removed from the documentation. Themers may wish to update any custom node templates to remove these references as well.