Tweak handling of nullable types in LUA doc generation.

The types for Int32 and Boolean are currently replaced with friendly names of int and bool for the docs. Ensure we apply the same handling when these are nullable types, changing the output from Int32? and Boolean? to int? and bool?

OpenRA.Game/Scripting/ScriptMemberExts.cs

@@ -32,7 +32,7 @@ namespace OpenRA.Scripting
 				ret = t.Name;
 
 			if (t.IsGenericType && t.GetGenericTypeDefinition() == typeof(Nullable<>))
-				ret = $"{t.GetGenericArguments().Select(p => p.Name).First()}?";
+				ret = $"{t.GetGenericArguments()[0].LuaDocString()}?";
 
 			return ret;
 		}