Document sync log limits

[tlovett1]

Sync logging is stored in options/post meta. This will pose problems on high usage websites. We need to document.

[peterwilsoncc]

@tlovett1 Am I correct in thinking your concern is that as the dt_sync_log option/meta data increases in size it may cause issues with performance? Either due to exceeding persistent cache sizes/being a huge chunk of data in the DB.

I know it's been a while since you wrote this, so it's no problem if you can't remember.