macOS: Document cases when sync badges do not appear on folders

[erikjv]

This "solves" owncloud/client#10250 . As written in that issue:

I asked Apple about this, see https://developer.apple.com/forums/thread/729720?answerId=753269022#753269022 .

Summary:

1. Finder expects that only one Sync Extension is attached to a folder, and
2. Yes, these folders are special to Finder, but
3. There is no documented list, because of item 1: there can be other Sync Extensions looking at a folder, and there is no way to find out about that.

I'm not sure if we should document the following: my guess is that the special folders for Finder are the ones with special folder icons, and the ones that can be synced with iCloud.

[sndbbbl]

There seems to be an improvement on this matter from Apple - all folders show the sync status now, also the "dedicated macOS folders", except the Documents folder itself. Desktop, Downloads, etc. and the subfolders of Documents show the sync status now, except the Documents folder itself...

So this is a big improvement to what was there before...