Unclear what time is mentioned...

agreed. Spent 10 mins to figure out mystical meaning behind the "window"

web/core/lib/Drupal/Core/Flood/FloodInterface.php says the following about this:

   * @param int $window
   *   (optional) Number of seconds in the time window for this event (default is 3600
   *   seconds, or 1 hour).

   * @param int $threshold
   *   The maximum number of times each user can do this event per time window.

So regarding your question:

Is this the time after the IP adres login limit is passed? I have the number of limits set to 5 and window 1 hour. So in this case: after 5 failes login attempts you cannot use this IP adres for 1 hour any more?
Or is it that the 5 times login in is counted within 1 hour?

It is the second: the time in which you are allowed to do an action (e.g. login with wrong password):
So 5 wrong password attempts in an hour will put you in the flood list and further attempts are being blocked.

Yes, I am all in favour of adding more and better help text (also for all the other fields) and a better readme.

Feel free to create a patch.

Attempted to clarify form message.

(I had problems using Gitlab (I'm use to Github workflow). I may have commited to the wrong branch.)

Looks good to me.

Automatically closed - issue fixed for 2 weeks with no activity.