Update form_types.rst
Update form_types.rst

If no admin class for the linked model class is defined no options are rendered in the form. It is really hard to debug this. I only found the solution when I found the following issue: See issue: https://github.com/sonata-project/SonataAdminBundle/issues/2012

Adding this note to the documentation could save people hours of debugging.
rschumacher/SonataAdminBundle,lightglitch/SonataAdminBundle,fosvn/SonataAdminBundle,Romashka/SonataAdminBundle,news-life-media/SonataAdminBundle,andythorne/SonataAdminBundle,claudusd/SonataAdminBundle,0mars/SonataAdminBundle,jszmidt/SonataAdminBundle,Romashka/SonataAdminBundle,Koc/SonataAdminBundle,sonata-project/SonataAdminBundle,ellementA/SonataAdminBundle,greg0ire/SonataAdminBundle,ryanhughes/SonataAdminBundle,ossinkine/SonataAdminBundle,news-life-media/SonataAdminBundle,dmarkowicz/SonataAdminBundle,morgangiraud/SonataAdminBundle,lutangar/SonataAdminBundle,mikemeier/SonataAdminBundle,jakublech/SonataAdminBundle,robhunt3r/SonataAdminBundle,gRoussac/SonataAdminBundle,jlamur/SonataAdminBundle,Denniskevin/SonataAdminBundle,FabienD/SonataAdminBundle,mgdigital/SonataAdminBundle,osavchenko/SonataAdminBundle,fosvn/SonataAdminBundle,mohebifar/SonataAdminBundle,robhunt3r/SonataAdminBundle,robhunt3r/SonataAdminBundle,nexylan/SonataAdminBundle,jordisala1991/SonataAdminBundle,mohebifar/SonataAdminBundle,rmzamora/SonataAdminBundle,rswork/SonataAdminBundle,rschumacher/SonataAdminBundle,dmarkowicz/SonataAdminBundle,gRoussac/SonataAdminBundle,cmacetko/SonataAdminBundle,greg0ire/SonataAdminBundle,dankempster/SonataAdminBundle,nexylan/SonataAdminBundle,binhle410/SonataAdminBundle,ossinkine/SonataAdminBundle,ejkun/SonataAdminBundle,dankempster/SonataAdminBundle,tim96/SonataAdminBundle,rmzamora/SonataAdminBundle,tskorupka/SonataAdminBundle,lutangar/SonataAdminBundle,Denniskevin/SonataAdminBundle,Koc/SonataAdminBundle,0mars/SonataAdminBundle,jszmidt/SonataAdminBundle,edouardkombo/SonataAdminBundle,digitalkaoz/SonataAdminBundle,Koc/SonataAdminBundle,binhle410/SonataAdminBundle,Th3Mouk/SonataAdminBundle,mikemeier/SonataAdminBundle,amine2z/SonataAdminBundle,tunne/SonataAdminBundle,mgdigital/SonataAdminBundle,Storke/SonataAdminBundle,dankempster/SonataAdminBundle,ruscon/SonataAdminBundle,cmacetko/SonataAdminBundle,goldbroker/SonataAdminBundle,arbislimen/SonataAdminBundle,vincenttouzet/SonataAdminBundle,Soullivaneuh/SonataAdminBundle,mweimerskirch/SonataAdminBundle,nexylan/SonataAdminBundle,andythorne/SonataAdminBundle,mohebifar/SonataAdminBundle,Storke/SonataAdminBundle,qsomazzi/SonataAdminBundle,ju1ius/SonataAdminBundle,tim96/SonataAdminBundle,FabienD/SonataAdminBundle,rschumacher/SonataAdminBundle,Romashka/SonataAdminBundle,ARTACK/SonataAdminBundle,ARTACK/SonataAdminBundle,gnorby88/SonataAdminBundle,Th3Mouk/SonataAdminBundle,UnDeleteRU/SonataAdminBundle,tunne/SonataAdminBundle,jordisala1991/SonataAdminBundle,lutangar/SonataAdminBundle,ellementA/SonataAdminBundle,j-ledoux/SonataAdminBundle,andythorne/SonataAdminBundle,goldbroker/SonataAdminBundle,rschumacher/SonataAdminBundle,osavchenko/SonataAdminBundle,amine2z/SonataAdminBundle,amine2z/SonataAdminBundle,rswork/SonataAdminBundle,jlamur/SonataAdminBundle,digitalkaoz/SonataAdminBundle,ARTACK/SonataAdminBundle,idchlife/SonataAdminBundle,Denniskevin/SonataAdminBundle,tim96/SonataAdminBundle,digitalkaoz/SonataAdminBundle,robhunt3r/SonataAdminBundle,Soullivaneuh/SonataAdminBundle,jlamur/SonataAdminBundle,ryanhughes/SonataAdminBundle,Th3Mouk/SonataAdminBundle,binhle410/SonataAdminBundle,claudusd/SonataAdminBundle,j-ledoux/SonataAdminBundle,claudusd/SonataAdminBundle,j-ledoux/SonataAdminBundle,edouardkombo/SonataAdminBundle,ARTACK/SonataAdminBundle,Romashka/SonataAdminBundle,joshlopes/SonataAdminBundle,dmarkowicz/SonataAdminBundle,edouardkombo/SonataAdminBundle,joshlopes/SonataAdminBundle,ksom/SonataAdminBundle,greg0ire/SonataAdminBundle,arbislimen/SonataAdminBundle,tunne/SonataAdminBundle,j-ledoux/SonataAdminBundle,Armstrong1992/SonataAdminBundle,vincenttouzet/SonataAdminBundle,mgdigital/SonataAdminBundle,ju1ius/SonataAdminBundle,nexylan/SonataAdminBundle,jordisala1991/SonataAdminBundle,mweimerskirch/SonataAdminBundle,lightglitch/SonataAdminBundle,binhle410/SonataAdminBundle,rmzamora/SonataAdminBundle,lightglitch/SonataAdminBundle,tskorupka/SonataAdminBundle,mohebifar/SonataAdminBundle,FabienD/SonataAdminBundle,Armstrong1992/SonataAdminBundle,cmacetko/SonataAdminBundle,Storke/SonataAdminBundle,digitalkaoz/SonataAdminBundle,news-life-media/SonataAdminBundle,idchlife/SonataAdminBundle,ksom/SonataAdminBundle,gRoussac/SonataAdminBundle,tskorupka/SonataAdminBundle,UnDeleteRU/SonataAdminBundle,arbislimen/SonataAdminBundle,ruscon/SonataAdminBundle,gnorby88/SonataAdminBundle,osavchenko/SonataAdminBundle,tim96/SonataAdminBundle,UnDeleteRU/SonataAdminBundle,ejkun/SonataAdminBundle,tunne/SonataAdminBundle,rmzamora/SonataAdminBundle,jlamur/SonataAdminBundle,ryanhughes/SonataAdminBundle,mikemeier/SonataAdminBundle,lightglitch/SonataAdminBundle,ruscon/SonataAdminBundle,morfin60/SonataAdminBundle,gnorby88/SonataAdminBundle,idchlife/SonataAdminBundle,Soullivaneuh/SonataAdminBundle,Soullivaneuh/SonataAdminBundle,dmarkowicz/SonataAdminBundle,andythorne/SonataAdminBundle,0mars/SonataAdminBundle,binhle410/SonataAdminBundle,OskarStark/SonataAdminBundle,ossinkine/SonataAdminBundle,amine2z/SonataAdminBundle,morgangiraud/SonataAdminBundle,idchlife/SonataAdminBundle,peter-gribanov/SonataAdminBundle,mgdigital/SonataAdminBundle,claudusd/SonataAdminBundle,jlamur/SonataAdminBundle,news-life-media/SonataAdminBundle,ellementA/SonataAdminBundle,osavchenko/SonataAdminBundle,rswork/SonataAdminBundle,jakublech/SonataAdminBundle,core23/SonataAdminBundle,ju1ius/SonataAdminBundle,ju1ius/SonataAdminBundle,qsomazzi/SonataAdminBundle,ruscon/SonataAdminBundle,morfin60/SonataAdminBundle,Th3Mouk/SonataAdminBundle,UnDeleteRU/SonataAdminBundle,ryanhughes/SonataAdminBundle,lutangar/SonataAdminBundle,peter-gribanov/SonataAdminBundle,Denniskevin/SonataAdminBundle,ossinkine/SonataAdminBundle,jszmidt/SonataAdminBundle,OskarStark/SonataAdminBundle,morgangiraud/SonataAdminBundle,rswork/SonataAdminBundle,gnorby88/SonataAdminBundle,jszmidt/SonataAdminBundle,fosvn/SonataAdminBundle,gRoussac/SonataAdminBundle,tskorupka/SonataAdminBundle,vincenttouzet/SonataAdminBundle,core23/SonataAdminBundle,Storke/SonataAdminBundle,ejkun/SonataAdminBundle,morfin60/SonataAdminBundle,edouardkombo/SonataAdminBundle,goldbroker/SonataAdminBundle,Romashka/SonataAdminBundle,ejkun/SonataAdminBundle,morgangiraud/SonataAdminBundle,peter-gribanov/SonataAdminBundle,OskarStark/SonataAdminBundle,ellementA/SonataAdminBundle,arbislimen/SonataAdminBundle,fosvn/SonataAdminBundle,core23/SonataAdminBundle,jordisala1991/SonataAdminBundle,Koc/SonataAdminBundle,0mars/SonataAdminBundle,UnDeleteRU/SonataAdminBundle,ksom/SonataAdminBundle,core23/SonataAdminBundle,joshlopes/SonataAdminBundle,j-ledoux/SonataAdminBundle,vincenttouzet/SonataAdminBundle,mikemeier/SonataAdminBundle,cmacetko/SonataAdminBundle,ejkun/SonataAdminBundle,osavchenko/SonataAdminBundle,morfin60/SonataAdminBundle,greg0ire/SonataAdminBundle,jakublech/SonataAdminBundle,qsomazzi/SonataAdminBundle,qsomazzi/SonataAdminBundle,ellementA/SonataAdminBundle,news-life-media/SonataAdminBundle,dankempster/SonataAdminBundle,OskarStark/SonataAdminBundle,sonata-project/SonataAdminBundle,mweimerskirch/SonataAdminBundle,jakublech/SonataAdminBundle,morfin60/SonataAdminBundle,ksom/SonataAdminBundle,peter-gribanov/SonataAdminBundle,joshlopes/SonataAdminBundle,goldbroker/SonataAdminBundle,mweimerskirch/SonataAdminBundle,dmarkowicz/SonataAdminBundle,FabienD/SonataAdminBundle