docs(tutorials): add fuel consumption rules explanation in generator tutorial

Author: GulinSS

docs/tutorials/06-beyond-fuel.md

@@ -134,6 +134,13 @@ genSomething Dry      = -- Base case: no fuel, choose non-recursive constructors
 genSomething (More f) = -- Recursive case: spend fuel or use optimization
 ```
 
+- When `Fuel` is `Dry`, the generator **must** choose non-recursive constructors (like `PZ`, `Leaf`, `Nil`)
+- When `Fuel` is `More f`, the generator **can** choose recursive constructors (like `PS`, `Node`, `Cons`)
+- Each recursive call consumes one unit of `Fuel` (calls with `f` instead of `More f`)
+- This guarantees termination: eventually `Fuel` reaches `Dry` and recursion stops
+
+This is why higher `Fuel` values produce larger structures — more recursive steps are allowed before hitting the `Dry` base case.
+
 But the key insight is: **deriveGen doesn't always spend fuel on recursion**. It depends on the type of recursion.
 
 ### SpendingFuel: Manual Implementation