Welcome to Waze and the forums.

You can just report it as a general Hazard, with or without the description. On iPhones people can use keyboard abbreviations for some common descriptions like ty=Thank You, lo=Traffic Lights Out etc. On Android, custom keyboards like SlideIT can do the same thing.
Waze keeps stats on how long it takes to travel from one segment of a road to another and also for each of the possible turns at intersections. In this case Waze would normally detect the extra time taken at the traffic light, and if it determines that it's quicker along another route it would reroute people around the problem area.
Just bear in mind that it is still sometimes quicker to go through a traffic jam than around it.

Regards