update form types documentation - fix #4397 (#4399)
update form types documentation - fix #4397 (#4399)

replace empty_value by placeholder in form definition
mweimerskirch/SonataAdminBundle,morfin60/SonataAdminBundle,jordisala1991/SonataAdminBundle,greg0ire/SonataAdminBundle,osavchenko/SonataAdminBundle,dmarkowicz/SonataAdminBundle,UnDeleteRU/SonataAdminBundle,Armstrong1992/SonataAdminBundle,sonata-project/SonataAdminBundle,OskarStark/SonataAdminBundle,dmarkowicz/SonataAdminBundle,mweimerskirch/SonataAdminBundle,morfin60/SonataAdminBundle,Soullivaneuh/SonataAdminBundle,jordisala1991/SonataAdminBundle,greg0ire/SonataAdminBundle,osavchenko/SonataAdminBundle,ossinkine/SonataAdminBundle,Armstrong1992/SonataAdminBundle,osavchenko/SonataAdminBundle,morfin60/SonataAdminBundle,greg0ire/SonataAdminBundle,ARTACK/SonataAdminBundle,dmarkowicz/SonataAdminBundle,Soullivaneuh/SonataAdminBundle,mweimerskirch/SonataAdminBundle,UnDeleteRU/SonataAdminBundle,sonata-project/SonataAdminBundle,morfin60/SonataAdminBundle,dmarkowicz/SonataAdminBundle,ARTACK/SonataAdminBundle,jlamur/SonataAdminBundle,OskarStark/SonataAdminBundle,peter-gribanov/SonataAdminBundle,osavchenko/SonataAdminBundle,Soullivaneuh/SonataAdminBundle,ARTACK/SonataAdminBundle,peter-gribanov/SonataAdminBundle,greg0ire/SonataAdminBundle,ossinkine/SonataAdminBundle,dmarkowicz/SonataAdminBundle,ossinkine/SonataAdminBundle,UnDeleteRU/SonataAdminBundle,UnDeleteRU/SonataAdminBundle,Soullivaneuh/SonataAdminBundle,mweimerskirch/SonataAdminBundle,UnDeleteRU/SonataAdminBundle,peter-gribanov/SonataAdminBundle,peter-gribanov/SonataAdminBundle,jordisala1991/SonataAdminBundle,jlamur/SonataAdminBundle,jlamur/SonataAdminBundle,jordisala1991/SonataAdminBundle,osavchenko/SonataAdminBundle,jlamur/SonataAdminBundle,morfin60/SonataAdminBundle,OskarStark/SonataAdminBundle,jlamur/SonataAdminBundle,OskarStark/SonataAdminBundle,ossinkine/SonataAdminBundle,ARTACK/SonataAdminBundle