Document automatic simul "unjoin" behavior

[TBestLittleHelper]

Context : https://lichess.org/forum/lichess-feedback/bug-simul-participants-disappear-after-registration

TL:DR : Simuls will unjoin players who leave the simul page - to avoid games where a player is not present and makes zero moves. We should inform about this somehow, so that the simul host is not confused.

• Probably ok if a message is only visible to the simul host. " Lichess will automatically remove players from the join list if ... "
• For simuls (such as the one in the forum) that are scheduled in advance with a starting time, we should consider tweaking the behavior. " In the last 5 minutes before the simul is scheduled to start Lichess will remove players if ... " and similarly shows a message to players "Make sure to be present on this page 5 minutes before the scheduled start"
• I guess there could be an option to disable to auto unjoin behavior's.. but honestly, I would opt for avoiding that and instead going for the other bullet points.

[maxturko]

Guys, this is horrible, we want to schedule a simul in advance but we encountered a situation where players were unjoined. We thought this was for no particular reason until we ended up here. I have some experience in Java development, maybe I could fix this, aka add a checkbox to control this "unjoin" behavior? But I am not sure where to start. Thanks in advance