Add comment on purpose of 2nd argument in angularFireCollection
Add comment on purpose of 2nd argument in angularFireCollection

bpietravalle/angularfire,fbentz/angularfire,denisKaranja/angularfire,itxd/angularfire,fbentz/angularfire,denisKaranja/angularfire,bpietravalle/angularfire,douglascorrea/angularfire,FirebaseExtended/angularfire,cesarmarinhorj/angularfire,FirebaseExtended/angularfire,itxd/angularfire,yoda-yoda/angularfire,cesarmarinhorj/angularfire,yoda-yoda/angularfire,itxd/angularfire,douglascorrea/angularfire,denisKaranja/angularfire,jamestalmage/angularfire,cesarmarinhorj/angularfire,jamestalmage/angularfire,nbr1ninrsan2/angularfire,yoda-yoda/angularfire,nbr1ninrsan2/angularfire,jamestalmage/angularfire,bpietravalle/angularfire,nbr1ninrsan2/angularfire,FirebaseExtended/angularfire,fbentz/angularfire,douglascorrea/angularfire