spraycan: actually explain SpawnDecal arg0 thing

zscript/spraycan.zs

@@ -405,9 +405,11 @@ class SnekTechSprayerDecalSpawner : Decal
             }
         }
 
-        // pass the decal name as a map editor type of argument
+        // SpawnDecal checks arg0 for the decal ID/name.
-        args[0] = -int(name(actualDecalName));
+        // gzdoom passes string arguments as negated string table indices, so
-        // spawn the decal
+        // we need to cast the decal name to a Name (implicitly adding it to
+        // the string table), then to an int (to get the index).
+        args[0] = -int(Name(actualDecalName));
         Super.SpawnDecal();
 
         // then find every decal that we spawned