I have to assume it is a bug, as it would be completely ridiculous for a game trying to be a competitive multiplayer game to be missing this much information.
There are no abilities at all that have tooltips showing what the ability actually does. This applies to things like the explosion part of the Ork Big Trakk and Eldar Fire Prism, the super charging of the Tactical Space Marine's Plasma Guns, the damage from Grenades, the duration of Stealth abilities...
Please fix this. It is ridiculous for players to have to go in game and manually test things out in order to determine how much damage these abilities do, or whether "Temporary Stealth" means 5 seconds or 45 seconds.