Art - Max Roll Angle and Maximum Pitch Angle: These two properties can be very helpful when used right. They allow you to tilt the doodads along the X and Y axis of the terrain. In other words: you can rotate your doodad in 3 dimensions.
The values work in radians, so if you want to give the doodad a half rotation (180 degrees), you have to set one of the values to -3.14. -1.57 (-3.14/2) would be a 90 degrees rotation, -6.28 (-3.14*2) a full rotation and so on.
Yes, the values have to be negative. (To insert negative values you have to allow negative values under File>Preferences and you need to press shift while double-clicking on the value.