Clarify stack conveyor recharge property

(also, should be minimum & multiple of 1)

core/src/mindustry/world/blocks/distribution/StackConveyor.java

@@ -27,6 +27,7 @@ public class StackConveyor extends Block implements Autotiler{
 
     public float speed = 0f;
     public boolean splitOut = true;
+    /** (minimum) amount of loading docks needed to fill a line */
     public float recharge = 2f;
     public Effect loadEffect = Fx.plasticburn;
     public Effect unloadEffect = Fx.plasticburn;