Update docs/discounts/extend_discounts.md

Co-authored-by: Paweł Niedzielski <[email protected]>

Author: mnocon

docs/discounts/extend_discounts.md

@@ -93,7 +93,7 @@ For performance reasons, it's recommended to:
 
 - use variables only for lightweight calculations
 - use functions for resource-intensive calculations (for example, checking customer's order history)
-- implement caching (for example, in-memory) for function results to avoid redundant calculations within a single discount, if the function is used multiple times
+- implement caching (for example, in-memory) for function results to avoid redundant calculations when multiple discounts expressions might use the function
 - specify the most resource-intensive conditions as the last to evaluate. As all conditions must be met for the discount to apply, it's possible to skip evaluating them if the previous ones won't be met
 
 In a production implementation, you should consider refactoring the `current_user_registration_date` variable into a `get_current_user_registration_date` function to avoid always loading the current user object and improve performance.