Make `Array.sort_custom` callable requirements more explicit for the `a == b` case

[harrison-munitz]

Your Godot version:
4.4

Issue description:
If the sorting function provided to sort_custom returns true when a == b (E.g. using <= instead of just <), you will get an error that reads:

bad comparison function; sorting will be broken

The existing documentation does technically describe the correct expected behavior of the callable, but it could stand to be more explicit. I propose adding the following sentence to the main part of the method description (before the first code-example) to make the expected behavior completely unambiguous:

If the order of the two elements does not matter (E.g. they are equal), it should return false.

URL to the documentation page (if already existing):
https://docs.godotengine.org/en/stable/classes/class_array.html#class-array-method-sort-custom