Add client_history_timeout documentation.

Redmine: #6669
discarding on the clients so imagine a case:
Your hub is down for 12h, had networking issues, hub was down, you did hd migration something that took quite a few hours of downtime for the hub. In meanwhile your agents still do their thing as expected. They accumulate reports on all the changes they make and as hub is down, those are not collected.

Then you get your hub online and it's starting picking up this multiple hour history reports. In practice we found that if it was down for too long (maybe 24h), then it can take few hours for the hub to get back to stable operational state for larger installation. In meanwhile it will try to put all those hours of reporting into postgres and this require time.

So you can set this timeout that would state that you rather care about hub betting back quickly up and reporting back rather than to have reports on what was going on on those machines in that downtime.

This is one of the usecases for this setting.