feat(config): add scanProgressFileLimit and improve scanProgressIntervalS

[danog]

Counterpart of https://github.com/syncthing/syncthing/pull/10221

[tomasz1986]

Can you change the line breaks, so that a) they end at the 75th character, and b) new sentences start in the same line as the previous ones?

I know that this file in particular is a mishmash of different styles, but this at least seems to be the common style for most of the recently written documentation. 😅

There are also some conflicts to fix that have emerged in the meantime.