add note about PDOM heading, update pdomOrder docs about ScreenView sections, https://github.com/phetsims/tasks/issues/1080
add note about PDOM heading, update pdomOrder docs about ScreenView sections, https://github.com/phetsims/tasks/issues/1080

phetsims/phet-info,phetsims/phet-info,phetsims/phet-info,phetsims/phet-info