Refactor Spin to MaximumSpinSpeed

Additionally, add descriptions to
other FallsToEarth properties.

OpenRA.Mods.Common/Traits/Air/FallsToEarth.cs

@@ -19,10 +19,17 @@ namespace OpenRA.Mods.Common.Traits
 	public class FallsToEarthInfo : ITraitInfo, IRulesetLoaded, Requires<AircraftInfo>
 	{
 		[WeaponReference]
+		[Desc("Explosion weapon that triggers when hitting ground.")]
 		public readonly string Explosion = "UnitExplode";
 
-		public readonly bool Spins = true;
+		[Desc("Limit the maximum spin (in facing units per tick) that can be achieved while crashing.",
+			"0 disables spinning. Negative values imply no limit.")]
+		public readonly int MaximumSpinSpeed = -1;
+
+		[Desc("Does the aircraft (husk) move forward at aircraft speed?")]
 		public readonly bool Moves = false;
+
+		[Desc("Velocity (per tick) at which aircraft falls to ground.")]
 		public readonly WDist Velocity = new WDist(43);
 
 		public WeaponInfo ExplosionWeapon { get; private set; }