fix: Update mutation guide (#265)

Author: phated

src/guide/mutation.md

@@ -40,10 +40,14 @@ count /= 25
 print(count) // 2
 ```
 
-These operators also return the assigned result, so you can use the new value immediately.
+These operators return `void` to help alleviate ambiguity and make code easier to understand—use the name of the binding to get its current value.
 
 ```grain
 let mut count = 7
 
-print(count += 9) // 16
+print(count += 9) // void
+
+// Instead, use
+count += 9
+print(count) // 25
 ```