In cf88cf2657d721c68055a284e8c498a18639f74c the eql function provided in
The context of ArrayHashMap was changed to also include the key index,
but this wasn't properly updated in the documentation.

Since a flat `usize` is unintuitive, I've tried to explain the function
of the parameter as best I can based on the original commit.

Finally, I didn't do an extensive search if this eql definition is
incorrectly stated anywhere outside of these 2 spots. But I somewhat
doubt an file outside of `array_hash_map` would