Simplify return statements.

Remove redundant ‘this’.
Remove unused using directives.
Simplify LINQ chains.
Add some trait property descriptions.
Add readonly where viable.
Add fullstops to some yaml descriptions.

OpenRA.Mods.D2k/Traits/Carryable.cs

@@ -101,7 +101,7 @@ namespace OpenRA.Mods.D2k.Traits
 				.Where(p => p.Actor.Owner == self.Owner && !p.Trait.IsBusy && p.Actor.IsInWorld)
 				.Select(h => h.Actor);
 
-			return WorldUtils.ClosestTo(carriers, self);
+			return carriers.ClosestTo(self);
 		}
 
 		// This gets called by carrier after we touched down