Update documentation for `findString`, `rfindString`
@@ -1408,8 +1408,9 @@ length of the substring). Note that the range generated is
### String search
-You can find the first (respectively, last) occurrence of a string, starting
+You can find the first (respectively, last) occurrence of a substring, starting
at a certain `index`, in another string in O(N*M) time.
+Returns `-1` if the substring is not found.
```k
syntax Int ::= findString ( haystack: String , needle: String , index: Int ) [function, hook(STRING.find)]
