The Kotlin reference is not supposed to be used for finding methods.
The way to go is to use auto completion in an IDE. If you type `myArray.sort<ctrl+space>`, you get all one needs, including `sort`, `sorted`, `sortBy` (which you needed).
The docs are arranged in a way that one reads them in the order they are written (at least the first half). I supposed that would take more than half an hour given at the competition.
I don't think Kotlin is the best choice for programming challenges like codeforces - they really don't let him shine (although I use it whenever I can). But if this is a valid case, we can collaborate and write a short tutorial for C/C++ to Kotlin programmers.