[doc] Document the Service Translation
[doc] Document the Service Translation

Add some sparse documentation around the Service Translation for
a vehicule journey's validity patterm.

More documentation can be found over confluence:
 - https://confluence.kisio.org/pages/viewpage.action?pageId=3768839
 - https://github.com/TeXitoi/pinot2015presenter/blob/master/pres/pinot2015presenter-pres.pdf

xlqian/navitia,CanalTP/navitia,Tisseo/navitia,Tisseo/navitia,Tisseo/navitia,Tisseo/navitia,xlqian/navitia,xlqian/navitia,Tisseo/navitia,xlqian/navitia,xlqian/navitia,CanalTP/navitia,CanalTP/navitia,CanalTP/navitia,CanalTP/navitia