docs(luhn-algorithm): clarify input handling and steps in the example

subjects/luhn_algorithm/README.md

@@ -5,8 +5,9 @@
 Create a function which checks if a number is valid per the Luhn formula.
 
 The function will receive a string and return a boolean.
-An empty string or a number with only one digit will be considered an invalid number.
-Spaces are accepted in the script but have to be stripped during the calculation (in other words they won't affect the result).
+
+**Invalid inputs:** An empty string or a number with only one digit are considered invalid.
+**Handling spaces:** Spaces are accepted but have to be ignored during the calculation (in other words they won't affect the result).
 
 The Luhn formula is used to check if a number is a valid credit card number and in some other scenarios where you need to check a number fast and without accessing a database.
 
@@ -23,7 +24,7 @@ So we will get:
 
 - `4539 3195 0343 6467`
 - `4_3_ 3_9_ 0_4_ 6_6_`: numbers to modify
-- `8_6_ 6_9_ 0_8_ 3_3_`: modified numbers
+- `8_6_ 6_9_ 0_8_ 3_3_`: modified numbers (for numbers over 9 we subtracted 9 already)
 - `8569 6195 0383 3437`: the new sequence of digits
 - `80`: the sum of all digits
 - `80` is evenly divisible by 10 so the result is `true`