docs(accordion): add note about ng-class
docs(accordion): add note about ng-class

- Adds note about custom class usage for the accordion-group

Closes #5317

292388900/bootstrap,angular-ui/bootstrap,Kpler/bootstrap,Foxandxss/bootstrap,icfantv/bootstrap,shootermv/bootstrap,shootermv/bootstrap,wesleycho/bootstrap,benhalverson/bootstrap,angular-ui/bootstrap,Servoy/bootstrap,fpipita/bootstrap,Kpler/bootstrap,looker/bootstrap,infowrap/bootstrap,wesleycho/bootstrap,looker/bootstrap,RobJacobs/bootstrap,vfiles/bootstrap,Foxandxss/bootstrap,burlhock/bootstrap,christor/bootstrap,fpipita/bootstrap,byronigoe/bootstrap,Servoy/bootstrap,292388900/bootstrap,mabi/bootstrap,caoruiy/bootstrap,vfiles/bootstrap,benhalverson/bootstrap,benhalverson/bootstrap,infowrap/bootstrap,caoruiy/bootstrap,mabi/bootstrap,BookingBug/bootstrap,christor/bootstrap,benhalverson/bootstrap,icfantv/bootstrap,RobJacobs/bootstrap,BookingBug/bootstrap,burlhock/bootstrap,byronigoe/bootstrap