Fix some machine construction bugs. (#18179)

Content.Shared/Stacks/SharedStackSystem.cs

@@ -167,6 +167,7 @@ namespace Content.Shared.Stacks
             amount = Math.Min(amount, GetMaxCount(component));
             amount = Math.Max(amount, 0);
 
+            // Server-side override deletes the entity if count == 0
             component.Count = amount;
             Dirty(component);
 

Content.Shared/Stacks/StackComponent.cs

@@ -28,6 +28,7 @@ namespace Content.Shared.Stacks
 
         /// <summary>
         ///     Set to true to not reduce the count when used.
+        ///     Note that <see cref="Count"/> still limits the amount that can be used at any one time.
         /// </summary>
         [DataField("unlimited")]
         [ViewVariables(VVAccess.ReadOnly)]