Draft: Changed DecayTreeTuple to FunTuple in the doc.

FunTuple is new ready to be used to text decay files. The instructions are changed accordingly.

Marked draft as the instructions work, but not with a released DaVinci. It needs v60r2, so this MR should wait for that.

Also, once v60r2 is out I should point to the documentation in lhcb/DaVinci!647 (merged) .