Made all projectile and warhead fields readonly

This came up while working on the new documentation generation and comparing the results to ORAIDE's own code parser.

OpenRA.Mods.Common/Projectiles/InstantHit.cs

@@ -35,7 +35,7 @@ namespace OpenRA.Mods.Common.Projectiles
 
 		[Desc("Scan radius for actors with projectile-blocking trait. If set to a negative value (default), it will automatically scale",
 			"to the blocker with the largest health shape. Only set custom values if you know what you're doing.")]
-		public WDist BlockerScanRadius = new WDist(-1);
+		public readonly WDist BlockerScanRadius = new WDist(-1);
 
 		public IProjectile Create(ProjectileArgs args) { return new InstantHit(this, args); }
 	}