Improve the Folder Sync Ownership documentation

[spkane]

Feature description

Add more OS-specific details in:

https://docs.syncthing.net/advanced/folder-sync-ownership.html#unix-implementation

that cover various recommended options for configuring this feature on multiple systems.

Problem or use case

The documentation is very sparse for this feature. It has a Unix implementation and Windows implementation sections, but makes no specific mention of Linux or macOS, despite the fact that the suggestion about using setcap only works on Linux, and will not work on macOS.

Alternatives or workarounds

Browsing the forums or figuring it out for oneself.